If a man has a reason eat a cheese omelette - but his reason may not be the reason

It is an error to compare truisms like “If a man wants to eat a cheese  omelette, then he generally will if the opportunity exists and no other desire overrides”

with a law that says how fast a body will fall from a 3rd floor window. It is an error, because in the latter case, but not the former, we can tell in advance whether the condition holds, and we know what allowance to make if it doesn’t.

For a reason to be the reason why one performs an action the reason must cause the action. For example, one has a reason to turn on the television, say, to watch one’s favorite TV show. But this need not be the reason why one turns on the television. This is because the above reason did not cause one to turn on the television. As Davidson puts it:

[S]omething essential has certainly been left out, for a person can have a reason for an action, and perform the action, and yet this reason migt not be the reason why he did it.