user10950: I was listening to a talk on YouTube about vipassana practice as taught by Goenka. The woman doing the talk said that the Buddha taught that every time a stimulus enters the sense door there is a sensation in the body and it is these sensations that we react to not the stimulus itself. I have tried the body scan technique quite a few times but not very successfully and so have ended practicing Mahasi Sayadaws technique instead which is focusing on the abdomen rising and falling.
I have to say I'm not very aware of these sensations that are happening to every stimulus. Of course when I sit and meditate I notice some sensations such as itching, pulsing, pain etc but as far as the ones that are the result of every stimulus entering I'm not sure how I go about noticing those before I react. 
Also my understanding is that Buddhism also sees thought as a sense door so therefore thought also causes these sensations. Is that correct?  In order to be a successful practitioner do I need to train myself to notice all these sensations? Is it good enough to just notice some like itching, trembling?  Do I need to understand what causes them? Or is noticing them enough?   Or do I need to notice the feeling tone associated?  Or both?

user-1: "The woman doing the talk said that the Buddha taught that every time a stimulus enters the sense door there is a sensation in the body and it is these sensations that we react to not the stimulus itself."
Let's take the stock description in the suttas as reference, using vision as example:

In dependence on the eye and forms, eye-consciousness arises. The meeting of the three is contact. With contact as condition, feeling comes to be [pleasant, painful, neither]; with feeling as condition, craving ...

Another stock passage is:

“With the cessation of that contact to be experienced as [pleasant, painful, neither], the corresponding feeling that arose in dependence on that contact ceases and subsides.

It should be clear that these are describing events going on through time:  First something happens, than another. The practice is about noticing them and how they relate to each other. That's why stimuli are the objects of the practice.
With that said...
A common interpretation here is that it's not all stimulus that reach a sense door that we come to be aware of. For example, often there are many sounds happening around us and we are completely unaware of them (as I was when writing this paragraph), despite them reaching our ears. In this case, it's only when ear-consciousness arise (which is dependent on the sound and on the ear) that there's opportunity for contact.
That contact is, grossly speaking, the moment of "awareness" or "attention" for the stimulus, it's when we notice it, when we know "it happening". The absence of that contact means we are oblivious to that particular stimuli at that particular time, even if it reached the sense door.
Another example is some bodily discomfort that we only notice much later, even though it was already present for some time. Without contact, no painful feeling came to be from that discomfort, until the moment of contact.
Keep in mind that giving attention to the discomfort at two different moments means two different consciousness, two different contacts, and two different painful feelings. Finally, I called it simply "discomfort" but they are also not one and not the same as the other.
From contact as condition, a proliferation of things come to be. One of them is the feeling (vedana), which may be followed by our underlying tendencies of reactions to them. This feeling (vedana) is the most bare painful/pleasant response to what has been contacted. But then, for example, this chain may trigger a certain past memory of something painful, which creates another painful feeling. Then, this painful feeling is a response not to the first stimuli, but to the memory, and so on.
"I have to say I'm not very aware of these sensations that are happening to every stimulus."
Perhaps picking the few that are more prominent might be helpful?
"Of course when I sit and meditate I notice some sensations such as itching, pulsing, pain etc."
These are perfect. Since you seem to be practicing mindfulness exercises, here's an excerpt of a sutta dealing with this practice:

“Bhikkhus, while a bhikkhu dwells thus, mindful and clearly comprehending, diligent, ardent, and resolute, if there arises in him a painful feeling, he understands thus: ‘There has arisen in me a painful feeling. Now that is dependent, not independent. Dependent on what? Dependent on just this contact. But this contact is impermanent, conditioned, dependently arisen. So when the painful feeling has arisen in dependence on a contact that is impermanent, conditioned, dependently arisen, how could it be permanent?’ He dwells contemplating impermanence in contact and in painful feeling, he dwells contemplating vanishing, contemplating fading away, contemplating cessation, contemplating relinquishment. As he dwells thus, the underlying tendency to aversion in regard to contact and in regard to painful feeling is abandoned by him.
-- SN 36.8

The point of that practice is to create a strong understanding and conviction about all and any stimuli that we may come to contact through our senses:

That they are impermanent
That they are not self
That they are unsatisfactory

A deeper understanding would involve being able to follow up on the chain of events for a particular stimulus: from the arising of consciousness, to contact, to feeling and, then, to craving, etc.
Finally, this practice also should lead to a certain degree of equanimity, as per the last sentence:

As he dwells thus, the underlying tendency to aversion in regard to contact and in regard to painful feeling is abandoned by him.

...which is an actual behavioral change, and a very practical benefit. It means that craving is also weakened.
"Also my understanding is that Buddhism also sees thought as a sense door so therefore thought also causes these sensations. Is that correct?"
Yes, mind is a sense door, thought is a sense object. With the arising of mind-consciousness, the meeting of the three is contact. With that contact as condition, painful, pleasant, or nor-painful-or-pleasant feeling arises.
"In order to be a successful practitioner do I need to train myself to notice all these sensations?"
If you notice only one and make it your experiment of investigation as above, that's already a success, isn't it?
Then, take the next one that comes up. As soon as you are comfortable with making the many incoming stimuli objects of analysis, go ahead. With samatha practices, it becomes easier and very natural to be able to direct the mind to a stream of stimuli, as we become more sensitive to our senses or...well, able to concentrate better.
"Is it good enough to just notice some like itching, trembling?"
I'd say it's good, but just noticing it's presence without the actual exercises as above is not good enough -- that is, in regard to the practice you are trying to do.
"Do I need to understand what causes them?"
By the suttas above, it's important to understand the conditions that make them arise, and the how they cease. Thus, we are taught in terms of the sequence of conditions above: consciousness, sense door, sense object, contact, feeling, etc.
This is because this is the very foundation that, according to the tradition, makes suffering come to be and makes it cease. Thus, by understanding them, the whole castle of suffering can be made to cease.

user11258: Just know that you felt something or noticed something into your five senses.for an example if you feel amgry just know that your are feeling angry. And then automatically you will give up that feeling(anger). Like that we should have a good comprehension about what we feel and notice into senses. In short just be aware of five senses. You do not want to spare a special time  and do that,but just do your day to day activities with a good comprehension and awareness. In other words do them conciously.If you do so you do not want to waste your time on those meditations that are taught everywhere.

user10293: Please focus your attention nothing but on breathing in and out at the very first attempt. As you concentrate more and more, you will know everything detail what occurring at these sense doors. Only the most distinct sensation you better note, after that just return back to breathing in and out. If you notice equal intensity sensation at different places, just focus one. Mind used to be working once at a time. Sometimes if you cannot note detail as itching, trembling because maybe it is so fast, just note "know it". Never think further what cause them or associated with which. Only then your concentration become sharpen in short time and you will know detail on whole body and then their changes.   

user8157: 
I was listening to a talk on YouTube about vipassana practice as taught by Goenka. The woman doing the talk said that the Buddha taught
  that every time a stimulus enters the sense door there is a sensation
  in the body and it is these sensations that we react to not the
  stimulus itself.

The Buddha taught it is a 'vedana' ('pleasant or painful feeling') at the sense doors that the mind reacts to rather than a physical sensation in the body. 
For example, a pleasant feeling creates the reaction of love/lust/greed; an unpleasant feeling creates the reaction of hatred/anger/irritation; and mixed feeling creates the reaction of confusion. 
(Note: Sensations in the body are 'sankhara/thought/stress formations' rather than 'vedana'). 
To know every 'vedana' at the sense doors is an extremely advanced practice and not a practise for formal foundational meditation development but an auxiliary practise.
The more formal foundational practise such as Anapanasati is developed, the more mindfulness at the sense door feelings can be practised. 

I have tried the body scan technique quite a few times but not very successfully and so have ended practicing Mahasi Sayadaws technique
  instead which is focusing on the abdomen rising and falling.

The Buddha taught Anapanasati (Mindfulness With/When Breathing). Mahasi Sayadaw method is closer to Anapanasati than the Goenka method. 
To be aware of bodily sensations is best done via awareness of breathing.

Also my understanding is that Buddhism also sees thought as a sense door so therefore thought also causes these sensations. Is that
  correct?

The mind is a sense door and thought is a sense object of this mind sense door. Yes. Thought is a sense object that can cause 'vedana' (pleasant & painful feelings). For example, you think of your job/work and a painful feeling arises, followed by emotions of hatred. 
However, again, to practise on this level, that is, knowing the feeling exactly when it arises after the arising of the thought, is very advanced. 

In order to be a successful practitioner do I need to train myself
  to notice all these sensations?

To be a Buddha, yes, the mind must notice all these feelings (vedana). But to successfully progress with practise, train in something more basic, such as Anapanasati. 

Do I need to understand what causes them? Or is noticing them
  enough? Or do I need to notice the feeling tone associated? Or both?

All the above. However, this is very advanced as I suggested. 
In summary, the Buddha did not teach the Goenka method & the Goenka method is an extreme interpretation of what the Buddha taught in Satipatthana.
Start with noticing the breathing flowing in & out of & moving in the body. Instead of rigid focus on the rise & fall the abdomen, maintain a more open relaxed awareness.

10950: <p>I was listening to a talk on YouTube about vipassana practice as taught by Goenka. The woman doing the talk said that the Buddha taught that every time a stimulus enters the sense door there is a sensation in the body and it is these sensations that we react to not the stimulus itself. I have tried the body scan technique quite a few times but not very successfully and so have ended practicing Mahasi Sayadaws technique instead which is focusing on the abdomen rising and falling.<br>
I have to say I'm not very aware of these sensations that are happening to every stimulus. Of course when I sit and meditate I notice some sensations such as itching, pulsing, pain etc but as far as the ones that are the result of every stimulus entering I'm not sure how I go about noticing those before I react. 
Also my understanding is that Buddhism also sees thought as a sense door so therefore thought also causes these sensations. Is that correct?  In order to be a successful practitioner do I need to train myself to notice all these sensations? Is it good enough to just notice some like itching, trembling?  Do I need to understand what causes them? Or is noticing them enough?   Or do I need to notice the feeling tone associated?  Or both?</p>
 -1: <p><em>"<strong>The woman doing the talk said that the Buddha taught that every time a stimulus enters the sense door there is a sensation in the body and it is these sensations that we react to not the stimulus itself.</strong>"</em></p>

<p>Let's take the stock description in the suttas as reference, using vision as example:</p>

<blockquote>
  <p>In dependence on the eye and forms, eye-consciousness arises. The meeting of the three is contact. With contact as condition, feeling comes to be [pleasant, painful, neither]; with feeling as condition, craving ...</p>
</blockquote>

<p>Another stock passage is:</p>

<blockquote>
  <p>“With the cessation of that contact to be experienced as [pleasant, painful, neither], the corresponding feeling that arose in dependence on that contact ceases and subsides.</p>
</blockquote>

<p>It should be clear that these are describing events going on through time:  First something happens, than another. The practice is about noticing them and how they relate to each other. That's why stimuli are the objects of the practice.</p>

<p>With that said...</p>

<p>A common interpretation here is that it's not all stimulus that reach a sense door that we come to be aware of. For example, often there are many sounds happening around us and we are completely unaware of them (as I was when writing this paragraph), despite them reaching our ears. In this case, it's only when ear-consciousness arise (which is dependent on the sound and on the ear) that there's opportunity for <em>contact</em>.</p>

<p>That contact is, grossly speaking, the moment of "awareness" or "attention" for the stimulus, it's when we notice it, when we know "it happening". The absence of that contact means we are oblivious to that particular stimuli at that particular time, even if it reached the sense door.</p>

<p>Another example is some bodily discomfort that we only notice much later, even though it was already present for some time. Without contact, no <em>painful feeling</em> came to be from that discomfort, until the moment of contact.</p>

<p>Keep in mind that giving attention to the discomfort at two different moments means two different consciousness, two different contacts, and two different painful feelings. Finally, I called it simply "discomfort" but they are also not one and not the same as the other.</p>

<p>From contact as condition, a proliferation of things come to be. One of them is the feeling (vedana), which may be followed by our underlying tendencies of reactions to them. This feeling (vedana) is the most bare painful/pleasant response to what has been contacted. But then, for example, this chain may trigger a certain past memory of something painful, which creates another painful feeling. Then, this painful feeling is a response not to the first stimuli, but to the memory, and so on.</p>

<p><em>"<strong>I have to say I'm not very aware of these sensations that are happening to every stimulus.</strong>"</em></p>

<p>Perhaps picking the few that are more prominent might be helpful?</p>

<p><em>"<strong>Of course when I sit and meditate I notice some sensations such as itching, pulsing, pain etc.</strong>"</em></p>

<p>These are perfect. Since you seem to be practicing mindfulness exercises, here's an excerpt of a sutta dealing with this practice:</p>

<blockquote>
  <p>“Bhikkhus, while a bhikkhu dwells thus, mindful and clearly comprehending, diligent, ardent, and resolute, if there arises in him a painful feeling, he understands thus: ‘There has arisen in me a painful feeling. Now that is dependent, not independent. Dependent on what? Dependent on just this contact. But this contact is impermanent, conditioned, dependently arisen. So when the painful feeling has arisen in dependence on a contact that is impermanent, conditioned, dependently arisen, how could it be permanent?’ He dwells contemplating impermanence in contact and in painful feeling, he dwells contemplating vanishing, contemplating fading away, contemplating cessation, contemplating relinquishment. As he dwells thus, the underlying tendency to aversion in regard to contact and in regard to painful feeling is abandoned by him.</p>
  
  <p>-- <a href="https://suttacentral.net/en/sn36.8" rel="nofollow noreferrer">SN 36.8</a></p>
</blockquote>

<p>The point of that practice is to create a strong understanding and conviction about all and any stimuli that we may come to contact through our senses:</p>

<ul>
<li>That they are impermanent</li>
<li>That they are not self</li>
<li>That they are unsatisfactory</li>
</ul>

<p>A deeper understanding would involve being able to follow up on the chain of events for a particular stimulus: from the arising of consciousness, to contact, to feeling and, then, to craving, etc.</p>

<p>Finally, this practice also should lead to a certain degree of equanimity, as per the last sentence:</p>

<blockquote>
  <p>As he dwells thus, the underlying tendency to aversion in regard to contact and in regard to painful feeling is abandoned by him.</p>
</blockquote>

<p>...which is an actual behavioral change, and a very practical benefit. It means that craving is also weakened.</p>

<p><em>"<strong>Also my understanding is that Buddhism also sees thought as a sense door so therefore thought also causes these sensations. Is that correct?</em>"</strong></p>

<p>Yes, mind is a sense door, thought is a sense object. With the arising of mind-consciousness, the meeting of the three is contact. With that contact as condition, painful, pleasant, or nor-painful-or-pleasant feeling arises.</p>

<p><em>"<strong>In order to be a successful practitioner do I need to train myself to notice all these sensations?</strong>"</em></p>

<p>If you notice only one and make it your experiment of investigation as above, that's already a success, isn't it?</p>

<p>Then, take the next one that comes up. As soon as you are comfortable with making the many incoming stimuli objects of analysis, go ahead. With samatha practices, it becomes easier and very natural to be able to direct the mind to a stream of stimuli, as we become more sensitive to our senses or...well, able to concentrate better.</p>

<p><em>"<strong>Is it good enough to just notice some like itching, trembling?</strong>"</em></p>

<p>I'd say it's good, but just noticing it's presence without the actual exercises as above is not good enough -- that is, in regard to the practice you are trying to do.</p>

<p><em>"<strong>Do I need to understand what causes them?</strong>"</em></p>

<p>By the suttas above, it's important to understand the conditions that make them arise, and the how they cease. Thus, we are taught in terms of the sequence of conditions above: consciousness, sense door, sense object, contact, feeling, etc.</p>

<p>This is because this is the very foundation that, according to the tradition, makes suffering come to be and makes it cease. Thus, by understanding them, the whole castle of suffering can be made to cease.</p>
 11258: <p>Just know that you felt something or noticed something into your five senses.for an example if you feel amgry just know that your are feeling angry. And then automatically you will give up that feeling(anger). Like that we should have a good comprehension about what we feel and notice into senses. In short just be aware of five senses. You do not want to spare a special time  and do that,but just do your day to day activities with a good comprehension and awareness. In other words do them conciously.If you do so you do not want to waste your time on those meditations that are taught everywhere.</p>
 10293: <p>Please focus your attention nothing but on breathing in and out at the very first attempt. As you concentrate more and more, you will know everything detail what occurring at these sense doors. Only the most distinct sensation you better note, after that just return back to breathing in and out. If you notice equal intensity sensation at different places, just focus one. Mind used to be working once at a time. Sometimes if you cannot note detail as itching, trembling because maybe it is so fast, just note "know it". Never think further what cause them or associated with which. Only then your concentration become sharpen in short time and you will know detail on whole body and then their changes.   </p>
 8157: <blockquote>
  <p><strong>I was listening to a talk on YouTube about vipassana practice as taught by Goenka. The woman doing the talk said that the Buddha taught
  that every time a stimulus enters the sense door there is a sensation
  in the body and it is these sensations that we react to not the
  stimulus itself.</strong></p>
</blockquote>

<p>The Buddha taught it is a 'vedana' ('pleasant or painful feeling') at the sense doors that the mind reacts to rather than a physical sensation in the body. </p>

<p>For example, a pleasant feeling creates the reaction of love/lust/greed; an unpleasant feeling creates the reaction of hatred/anger/irritation; and mixed feeling creates the reaction of confusion. </p>

<p>(Note: Sensations in the body are '<em>sankhara/thought/stress formations</em>' rather than '<em>vedana</em>'). </p>

<p>To know every 'vedana' at the sense doors is an extremely advanced practice and not a practise for formal foundational meditation development but an auxiliary practise.</p>

<p>The more formal foundational practise such as Anapanasati is developed, the more mindfulness at the sense door feelings can be practised. </p>

<blockquote>
  <p><strong>I have tried the body scan technique quite a few times but not very successfully and so have ended practicing Mahasi Sayadaws technique
  instead which is focusing on the abdomen rising and falling.</strong></p>
</blockquote>

<p>The Buddha taught Anapanasati (Mindfulness With/When Breathing). Mahasi Sayadaw method is closer to Anapanasati than the Goenka method. </p>

<p>To be aware of bodily sensations is best done via awareness of breathing.</p>

<blockquote>
  <p><strong>Also my understanding is that Buddhism also sees thought as a sense door so therefore thought also causes these sensations. Is that
  correct?</strong></p>
</blockquote>

<p>The mind is a sense door and thought is a sense object of this mind sense door. Yes. Thought is a sense object that can cause 'vedana' (pleasant &amp; painful feelings). For example, you think of your job/work and a painful feeling arises, followed by emotions of hatred. </p>

<p>However, again, to practise on this level, that is, knowing the feeling exactly when it arises after the arising of the thought, is very advanced. </p>

<blockquote>
  <p><strong>In order to be a successful practitioner do I need to train myself
  to notice all these sensations?</strong></p>
</blockquote>

<p>To be a Buddha, yes, the mind must notice all these feelings (vedana). But to successfully progress with practise, train in something more basic, such as Anapanasati. </p>

<blockquote>
  <p><strong>Do I need to understand what causes them? Or is noticing them
  enough? Or do I need to notice the feeling tone associated? Or both?</strong></p>
</blockquote>

<p>All the above. However, this is very advanced as I suggested. </p>

<p>In summary, the Buddha did not teach the Goenka method &amp; the Goenka method is an extreme interpretation of what the Buddha taught in Satipatthana.</p>

<p>Start with noticing the breathing flowing in &amp; out of &amp; moving in the body. Instead of rigid focus on the rise &amp; fall the abdomen, maintain a more open relaxed awareness.</p>
