The sentence goes as follow:
Being rationally uncommitted does not do away with all the other sources of motivation that get us by — habit, desire, fear of the law and so on.
I googled the phrase and got this: get by means "to be able to live or deal with a situation with difficulty, usually by having just enough of something you need, such as money".
It seems slightly odd to adapt the above definition to this sentence. Is this the case? What does it mean by "motivations that get us by"?