I hold an apple in my hand realizing that the apple in my hand which I am perceiving is not the apple that exists outside of me, but rather exists as an internal abstraction of the apple itself. Therein begins the terrifying realization that everything I know or think I know is only that which I have perceived or learned, is only that which has been interpreted by my sense organs, filtered by them, reflected into my brain, filtered by my brain, and finally perceived by my brain.
Nothing I know is objective, only that which has been perceived. Perception of our higher reality, while being of this plane of existence, without filtration, is impossible. The apple to me exists as an abstraction, and thus as a concept. It exists as that which is, though material in nature, idealist to me. The apple is certainly infinite in scope, down to the smallest subatomic particles it is mathematically possible to go down in our analysis of the size of the parts of the apple forever. But it exists in actuality as a finite infinity. For to perceive the apple in its entirety is impossible, only a part can be perceived by us. Even this merely implies an analysis the apple as it exists in a moment frozen in time, not even taking into account the constant dialectic of decay and change in which the apple is undergoing.
What about who I am to myself? I must filter my perception of myself as well even though it is myself. And those around me I love and care about? They too are but abstractions who I can never truly know. In this I am utterly alone, in this, though together, we are all alone. But in realizing this it also comes to mind that, even the good things about life are also internalized reflections of the external, and that they too are a part of me in some small way. Entering into a field of extreme skepticism one can easily come to idealist conclusions that I am the only thing that exists and the world around me is merely a figment of my imagination. But one must accept a scientific analysis if one wishes to retain sanity. The external is real and the mind is an emanation of the material, though no doubt highly organized. But in a spirit of skepticism I can never really know the answer to this question, or the objective answer to any question. In short, the only thing that I know is that I know nothing, nothing at all. And this too is but an abstraction.