Experience, Emotions, and Learning.
I may be justified in believing that there is a tree in front of me by virtue of the fact that I am currently having a certain kind of sense experience, but this will be true only in “favorable circumstances.” If I am confronted with a complicated arrangement of mirrors, I may not be justified in believing that there is an oak tree in front of me, even though it looks for all the world as if there is. Again, it may look for all the world as if water is running uphill, but the general improbability of this greatly diminishes the justification the corresponding belief receives from that experience. (1)
As William P. Alston indicates in the quote above, most often we are perfectly justified in claiming that what we are seeing is indeed something of real substance before us. Yet, this is what Alston qualifies as a favorable circumstance. In other words, there is no reason to believe that I am hallucinating, dreaming, that the objects I see are holograms, and so forth. Favorable circumstances are just part of every-day-life. We seldom (unless you’re a philosopher working too much in metaphysics and epistemology) give much thought to the physical reality of the world we live in. When I reach out my hand for my mug of coffee, I fully expect that my fingers will grip the handle and I will be able to pick it up and bring it to my lips. I do not even think about whether my mug is real or not. (2)