Every aspect of experience begs this question, how do I know this is real? How do I know I am experiencing what my brain says I am? What if everything I see and love and enjoy is an illusion of senses? This uncertainty can be shown through the very simple act of experiencing something physical, like a potato. We experience this with primary and secondary qualities. The hardness or roundness of this potato seems the same for everybody, but then we look at taste and smell and touch and suddenly this is left up to interpretation and opinion. The secondary qualities are therefore better described as beliefs than knowledge. I know the potato tastes bad to me, but I may appreciate others' tastes are different. I know the potato feels rounded and hard, and although this is more certain than the secondary qualities, it is still an uncertain. I do not know how this potato truly is, as I am limited to one perspective. The empirical senses perhaps are not so reliable. .
Rationalism, for one, infers that all we experience through our empirical senses is deemed untrustworthy. We must only rely on pure reason and not accept anyone else who tries to convince us of their truth, because it is not the truth, only that which is immutable and undoubted can be true. Descartes was in favor of this theory. He proposed that if we can doubt something then it is not necessarily true and so everyday things around us therefore are simply an illusion. You can doubt that you are reading these words, you can doubt that I have written them, and as I feel my hands typing I may not be typing. This suggests that yes, through methodological doubt I may question most of my every day experiences. Descartes did state that there are things we cannot doubt, like our own existence from a sense that we are conscious in some form or another. .
The contradiction of this is known as empiricism, to trust your senses and believe that what is experienced is correct.