user-1: This philosophical treatment of Nagarjuna by Westerhoff talks about how a cause depends on its effect.
I think that this point is a stumbling block for me but in my philosophical interpretations of Buddhism and of life and death per se.
He says:

There are three different ways in which we can make sense of
  Nāgārjuna's assertion that the cause depends existentially on the
  effect.

And then proceeds to argue about them (which I cannot make complete sense of).
Is the idea that a conditioned cause must be conditioned by its effect, in the sense of having it as a part?

user4722: Approaching causal and effectual reliance through "times"
Past, Present, and Future are the three times.
Future, Past, and Present equally depend on each other.
If you try and draw a triangle with only one line or two lines, it is impossible.  All three need to be in place for the totality of "time."
In this way, it might be possible to reason more clearly about how causes depend on effects.  
Approaching co-emergent phenomena via phor
Your question seems to be "okay causes lead to effects, but why do causes depend on effects?"
Two people high-fiving requires a silent clap from both sides. 
The beach (of sand) and the shore-line (of water) depend upon one another.  To say there can be a beach without water is to have a desert.  To have water without a shore-line is not a shore, but an ocean.
Thus, investigating
The nature of where they meet
Noble Nagarjuna, in Fundamental Wisdom of the Middle Way states

Forms do not meet and they do not part.

Dogen-ji made a statement of similar magnitude by saying "Space has no gaps."
I will stop there, at risk of becoming too philosophical.  The teachings are meant for experiential understanding.  Words will help, but like reading a menu or eating a meal, the actual experience is what counts.
For now, know that the present is dominant and that both future and past must borrow from Now in order to even come into question.

user-1: Good question.
I have this magnificent book "How to See Yourself As You Really Are" by Dalai Lama. 
And in this book he actually speaks about the question you are asking about - how a cause is dependent on its effect.
Dalai Lama explains it brilliantly in his book so i am going to quote him directly. 
Both passages are from "Chapter 20: Notice How everything Depends on Thought, p. 201-203".

"For instance, it is obvious that effects depend upon causes, but causes also, in a subtle sense, depend upon effects. Every cause itself is an effect of its own causes, which preceded it, and therefore arises in dependence upon its respective causes. All Buddhist systems assert that effects arise in dependence upon causes. Here cause and effect are in a temporal sequence, an effect occurring after its cause. This is dependent-arising in the sense of dependent production. Only the highest philosophical perspective within Buddhism contains an additional consideration, that because the designation of something as a “cause” depends upon consideration of its effect, in this sense a cause depends upon its effect. Something is not a cause in and of itself; it is named a “cause” in relation to its effect. Here the effect does not occur before its cause, and its cause does not come into being after its effect; it is in thinking of its future effect that we designate something as a cause. This is dependent-arising in the sense of dependent designation."

He says further;

"As Nagarjuna says in his Fundamental Treatise on the Middle Called “Wisdom”:
  A doer is dependent on a doing, And a doing exists dependent on a doer. Except for dependently arising, we do not see another cause for their establishment. Agent and action depend upon each other. An action is posited in dependence upon an agent, and an agent is posited in dependence upon an action. An action arises in dependence upon an agent, and an agent arises in dependence upon an action. Nevertheless, they are not related in the same way as cause and effect, since the one is not produced before the other. How is it that, in general, things are relative? How is it that a cause is relative to its effect? It is because it is not established in and of itself. If that were the case, a cause would not need to depend on its effect. But there is no self-sufficient cause, which is why we do not find anything in and of itself when we analytically examine a cause, despite its appearance to our everyday mind that each thing has its own self-contained being. Because things are under the influence of something other than themselves, the designation of something as a cause necessarily depends upon consideration of its effect. This is the route through which we come to realize that this more subtle understanding of dependent-arising as dependent designation is correct."

Lanka

-1: <p>This philosophical treatment of <a href="http://plato.stanford.edu/entries/nagarjuna/#Cau" rel="nofollow">Nagarjuna by Westerhoff</a> talks about how a cause depends on its effect.</p>

<p>I think that this point is a stumbling block for me but in my philosophical interpretations of Buddhism <em>and</em> of life and death per se.</p>

<p>He says:</p>

<blockquote>
  <p>There are three different ways in which we can make sense of
  Nāgārjuna's assertion that the cause depends existentially on the
  effect.</p>
</blockquote>

<p>And then proceeds to argue about them (which I cannot make complete sense of).</p>

<p>Is the idea that a conditioned cause must be conditioned by its effect, in the sense of having it as a part?</p>
 4722: <h2>Approaching causal and effectual reliance through "times"</h2>

<p>Past, Present, and Future are the three times.</p>

<p>Future, Past, and Present equally depend on each other.</p>

<p>If you try and draw a triangle with only one line or two lines, it is impossible.  All three need to be in place for the totality of "time."</p>

<p>In this way, it might be possible to reason more clearly about how causes depend on effects.  </p>

<h2>Approaching co-emergent phenomena via phor</h2>

<p>Your question seems to be "okay causes lead to effects, but why do causes depend on effects?"</p>

<p>Two people high-fiving requires a silent clap from both sides. </p>

<p>The beach (of sand) and the shore-line (of water) depend upon one another.  To say there can be a beach without water is to have a desert.  To have water without a shore-line is not a shore, but an ocean.</p>

<p>Thus, investigating</p>

<h2>The nature of where they meet</h2>

<p>Noble Nagarjuna, in Fundamental Wisdom of the Middle Way states</p>

<blockquote>
  <p>Forms do not meet and they do not part.</p>
</blockquote>

<p>Dogen-ji made a statement of similar magnitude by saying "Space has no gaps."</p>

<p>I will stop there, at risk of becoming too philosophical.  The teachings are meant for experiential understanding.  Words will help, but like reading a menu or eating a meal, the actual experience is what counts.</p>

<p>For now, know that the present is dominant and that both future and past must borrow from Now in order to even come into question.</p>
 -1: <p>Good question.</p>

<p>I have this magnificent book <a href="http://ecx.images-amazon.com/images/I/41uUANX4j4L._SY344_BO1,204,203,200_.jpg" rel="nofollow">"How to See Yourself As You Really Are"</a> by Dalai Lama. </p>

<p>And in this book he actually speaks about the question you are asking about - <strong><em>how a cause is dependent on its effect</em></strong>.</p>

<p>Dalai Lama explains it brilliantly in his book so i am going to quote him directly. </p>

<p>Both passages are from <em>"Chapter 20: Notice How everything Depends on Thought, p. 201-203"</em>.</p>

<blockquote>
  <p>"For instance, it is obvious that effects depend upon causes, but causes also, in a subtle sense, depend upon effects. Every cause itself is an effect of its own causes, which preceded it, and therefore arises in dependence upon its respective causes. All Buddhist systems assert that effects arise in dependence upon causes. Here cause and effect are in a temporal sequence, an effect occurring after its cause. This is dependent-arising in the sense of dependent production. Only the highest philosophical perspective within Buddhism contains an additional consideration, that because the designation of something as a “cause” depends upon consideration of its effect, in this sense a cause depends upon its effect. Something is not a cause in and of itself; it is named a “cause” in relation to its effect. Here the effect does not occur before its cause, and its cause does not come into being after its effect; it is in thinking of its future effect that we designate something as a cause. This is dependent-arising in the sense of dependent designation."</p>
</blockquote>

<p>He says further;</p>

<blockquote>
  <p>"As Nagarjuna says in his Fundamental Treatise on the Middle Called “Wisdom”:
  A doer is dependent on a doing, And a doing exists dependent on a doer. Except for dependently arising, we do not see another cause for their establishment. Agent and action depend upon each other. An action is posited in dependence upon an agent, and an agent is posited in dependence upon an action. An action arises in dependence upon an agent, and an agent arises in dependence upon an action. Nevertheless, they are not related in the same way as cause and effect, since the one is not produced before the other. How is it that, in general, things are relative? How is it that a cause is relative to its effect? It is because it is not established in and of itself. If that were the case, a cause would not need to depend on its effect. But there is no self-sufficient cause, which is why we do not find anything in and of itself when we analytically examine a cause, despite its appearance to our everyday mind that each thing has its own self-contained being. Because things are under the influence of something other than themselves, the designation of something as a cause necessarily depends upon consideration of its effect. This is the route through which we come to realize that this more subtle understanding of dependent-arising as dependent designation is correct."</p>
</blockquote>

<p>Lanka</p>
