Read the Metaphysics -- that's what it's about. For Harry, it was the principle of non-contradiction:
the same proposition cannot be true and not true at the same time in the same way.
Without that, there can be no discussion.
Now, you'll need a concept of what it means "to be", what "true" is, as well as constitutes "knowledge".
(and, if you guessed that, for Harry, Scientific Knowledge consisted in principles from which the rest of the science could be derived, like a point and extension for geometry, or a lever, pulley and inclined plane for mechanics, well, you're own your way)
target's not too far from the truth. Good wine, good food and lots of leisure time is what gave birth to philosophia.