There are a thousand familiar disputes which reason can never decide; questions which elude investigation, and make logic ridiculous; cases where something must be done, and where little can be said. Consider the state of mankind, and enquire how few can be supposed to act upon any occasions, whether small or great, with all the reasons of action present to their minds. Wretched would be the pair above all names of wretchedness, who should be doomed to adjust by reason, every morning, all the minute detail of a domestic day.
Now, these words are spoken by a character in Johnson's Rasselas, and they are spoken with specific reference to married life. But I think they can reasonably be assumed to express the author's own view, and are well applied to the general state of humans. The point stands and Johnson is right: it is impossible to act rationally. It is foolish to suppose that you can.
To give one reason for this, consider the motive for our actions. Aren't people's actions ultimately guided by their hopes and dreams for the future, and aren't hopes and dreams themselves inherently irrational things? One could make a case that ants are far more rational than people, since they spend all their energy doing whatever it is that ants do, eat when they are hungry, sleep (do they?) when they are tired, and then get back to working at fulfilling the natural state of being an ant. What is it that humans do? How does a rational human occupy his or her waking hours?
It should be said, though, that the ant's superhuman capacity for rational behaviour only extends to a narrowly-defined environment. Take an ant out of the colony and put it in a library, say, and it will carry on trying to act the same way. But the behaviour that was rational in the creature's intended state becomes futile in the library, or anywhere else. Perhaps, then, the impossibility of rational action for humans derives from our having left, or gone beyond, nature-type living conditions. In fact, if you should find yourself abandoned in the jungle with no water or iPhone, then you might find that it becomes a lot easier to make strictly rational choices about how to behave, since the survival imperative then overrides any other considerations.
However, there is a problem with this view: by that argument, the most rational human alive (or more precisely, the human with the greatest capacity for rational action) would be Ray Mears. I have a sense that this cannot be right; so I am going to conclude that my line of argument has taken a wrong turning. But I am more right than the babbling Dawkinists and the bellowing Creationists. Repeal the rainbow, bring on the flood.
PS The other title I considered for this post was "Ant and deck".