Passages from Sartre’s Being and Nothingness always reverberate through my mind:  “Consciousness is what is not and is not what it is”.  “Consciousness is a being such that in its being its being is in question insofar as its being always implies a being other than itself”.  I remember the happy days reading this tom(e)(b) when I was young; diagraming these sentences, trying to decipher them like Zen koans.  I remember later reading Frege’s Foundations of Arithmetic.  “Zero is the number non-identical to itself”.  How many zeroes are there?  Many!  But that can’t be right.  By Leibniz’s principle of indiscernibles, two things must always be distinguished by something.  Yet zero is nothing.  There can only be one zero.  All zeroes must be the same.  But if there were one zero, then zero would be something.  A paradox.  No wonder zero was received as a heresy in…