Why φ? Here's why

Those who are new to debates about practical rationality may find it annoying that we usually use φ and Ψ when we want schematic letters with which to discuss actions, rather than using p or q or f or any of the other schematic letters familiar from other areas of philosophy. φ and Ψ are harder to create on your keyboard and beginners who don't know any Greek often don't even know what they are. (Regarding the keyboard issue, I usually just search 'phi' or 'psi' in Google, then copy and paste.)

There's a good reason why those of us working in practical philosophy use these letters, though. It's fine to talk about believing that p, but when you talk about an agent p-ing, it sounds like he or she is urinating. Consider this passage from Bernard Williams' "Internal and External Reasons", in which I have replaced all the φs with p's:

But we should notice that an unknown element in S, D, will provide a reason for A to p only if p-ing is rationally related to D; that is to say, roughly, a project to p could be the answer to a deliberative question formed in part by D. If D is unknown to A because it is in the unconscious, it may well not satisfy this condition, although of course it may provide the reason why he p's, that is, may explain or help to explain his p-ing. In such cases, the p-ing may be related to D only symbolically.

If philosophers were forced to read passages like this out loud at conferences, juvenile tittering would interrupt everyone's train of thought and no progress on substantial questions about practical rationality could ever be made. (The letter f, it should be mentioned, would cause its own problems.)

Perhaps those in other areas of philosophy would be wise to use Greek letters. I've been told of a lecture on some topic at the intersection of metaphysics and the philosophy of language which collapsed into uncontrollable laughter when a p-ness entered into a complex relation with an a-ness.