I suppose I should've emphasized the "on such a foundational level" -part of that sentence. Questioning and refining observations is obviously of paramount importance, but that's only valid if we assume that deriving knowledge about the nature of reality is at all possible via our senses and observations.
That's where the distinction between physics and metaphysics comes in. Metaphysics is philosophy and thus inherently unverifiable.
The things we imagine do exist, as patterns of activity in our brains, emerging from the complexity of a whole bunch of neurons in brains and as part of societies. I said as much in a previous comment about emergent materialism.