No mere expression of opinion counts as philosophy, as Plato was wont to remind us. One minimal necessary (but not sufficient) identifying mark of philosophy seems to me to be the recognition of at least some questions as genuine questions — that is to say, questions for which we explicitly acknowledge that we do not have immediate answers.
I also have a candidate for a necessary and sufficient condition. It now seems to me that all the questions that have traditionally been regarded as philosophical can be interpreted as at least indirectly having normative import, regardless of whether all those discussing them thought in that way. So we could say philosophy as a practice is the recognition of questions with normative import as genuine questions (and this is the way to the good life for a rational animal).
By this definition, we should expect to find no philosopher in any time exemplifying the attitude that all normative questions are already settled. I believe this is also true for all those who have in fact been commonly called philosophers in any serious sense (but see Antiphilosophy). If modernity is defined typologically as any step away from the attitude that all normative questions are already settled, then all philosophy would be “modern” in this somewhat unusual sense. (I’m not a big fan of the pre-Socratics, but I do think they fit this description. Most serious theology through the centuries has been enough influenced by philosophy to recognize that there are genuine normative questions, and in that measure I count it too as philosophy.) (See also History of Philosophy.)
I’m almost tempted to suggest substituting “philosophy” for “modernity” in the discussion of the history of normativity. But there may be unphilosophical modernity. The Sophists strike me as “modern” under this criterion and I don’t consider them philosophers, since I am taking Plato and Aristotle at their word that the Sophists claimed either to have all the answers or that there were no real answers. (I have not examined recent literature on the Sophists, some of which I believe argues for a different assessment.)
I suspect the first glimmerings of typological modernity (as distinct from philosophy) go back at least as far as the first cities, and possibly as far as the relatively long-distance trade in the late Upper Paleolithic that began to put people raised in traditional attitudes face-to-face with others reared with different traditional attitudes. However, Aristotle and Hegel would remind us that fully fledged forms are more relevant than origins for most purposes. (See also Interpretation; Ethical Reason.)