Principles of Philosophy
(...in no particular order. Feel free to email me with suggestions!)
1. Every one's problem is no one's problem.
2. Proofs of non-existence are tricky.
3. If you interpret someone smart as saying something stupid, you might want to evaluate your interpretation. (This is often called the principle of charity.)
4. Everything is what it is, and not another thing (Attributed to Butler by Moore).
5. To say of what is that it is not, or of what is not that it is, is false; while to say of what is that it is, and of what is not that it is not, is true (Aristotle, Metaphysics 1011b25).
6. Whereof one cannot speak, thereof one must be silent (Wittgenstein, Tractatus 7.)