Dirac left me in the dust at this step. Can someone please help me through it. I understand from Susskind's lectures that if we need a constant, that we can assign any label to it, and that i[itex]\hbar[/itex] is merely a prescient choice. But I fail to understand Dirac's logic of why "[itex]\hbar[/itex] must simply be a number." Also, am I right in feeling that HUP just appeared out of nowhere in this mysterious algebraic step? If so, Dirac didn't call attention to it in this chapter. p.s. I apologize for a previously fumbled attempt to post this question.