Lazy reason
Lazy Reason is a pejorative name for a logical argument which holds that since all events are predestined, it is not necessary to carefully deliberate about one's actions.

Determinism is the philosophical proposition that every event, including human cognition and action, is causally determined by an unbroken chain of prior occurrences.  In other words, Determinism holds that Free Will does not exist, and that everything is essentially pre-determined.

But this presents a puzzle:  If everything is pre-determined, why should we worry about the future, if it is something we cannot change?

"Lazy Reason" is the idea that, indeed, we should not worry about the future, and should instead just accept whatever the future holds.

Historical Discussion
One of the most vocal opponents of "lazy reason" was the philosopher Leibniz.  In his preface to Theodicy, Leibniz discussed "lazy reason":

Men have been perplexed in well-nigh every age by a sophism which the ancients called the 'Lazy reason', because it tended towards doing nothing, or at least towards being careful for nothing, and only following inclination for the pleasure of the moment. For, they said, if the future is necessary, that which must happen will happen, whatever I may do. Now the future (so they said) is necessary, whether because the Divinity foresees everything, and even pre-establishes it by the control of all things in the universe; or because everything happens of necessity, through the concatenation of causes; or finally, through the very nature of truth, which is determinate in the assertions that can be made on future events, as it is in all assertions, since the assertion must always be true or false in itself, even though we know not always which it is. And all these reasons for determination which appear different converge finally like lines upon one and the same center; for there is a truth in the future event which is pre-determined by the causes, and God pre-establishes it in establishing the causes.

In his Discourse on Metaphysics, Leibniz also says:
"One should not be a quietist with the arms folded, open to ridicule, awaiting that which God will do; according to the sophism which the ancients called 'logon aergon', the lazy reason."

A modern example

One near contemporary presentation of lazy reason is the theme of the 1956 popular song "Que Sera, Sera (Whatever Will Be, Will Be)".  In the song, a young child repeatedly asks questions about her future, but is advised that "Whatever will be, will be":

When I was just a child in school, I asked my teacher, what should I try?
Should I paint pictures? Should I sing songs? This was her wise reply:
Que sera sera-- Whatever will be, will be.  The future's not ours to see;
So, que sera sera-- What will be, will be.