It is not my intent to go into the physics of seeing in any great detail. Any standard textbook, or internet equivalent, on the subject will perform that task quite adequately. There are, however, certain points I would like to emphasise. Light from an outside object (the word 'outside' yet to be defined) enters the eye but goes no further than the retina. At that point the light energy is converted into signals that are recognised and processed by the brain. Even though the brain is in darkness, it is able to recognise or 'see' light. However, it is the mind that is the final arbiter of what lies outside, whether or not the brain agrees. Therefore, from the mind's perspective, everything which is not part of itself, lies outside itself. Thus everything that is part of the material universe can be considered to be 'outside', including the brain mass.
Being the creatures that we are, our vision is limited to a very narrow range of electromagnetic wavelengths. (400-700 nm, or 0.4-0.7 millionths of a metre, compared with up to hundreds of metres for radio waves, and 10 billionths of a metre for gamma rays.) The point here is that we see only a tiny fraction of what is available out there. And not only is our vision very limited, but most of what we do see is ignored (or can be considered to be blind to) anyway, depending on our prioritising processes.
Consider now the structure of the matter we see around us. It is composed of a very tiny fraction of what might be considered to be 'solid', using the Niels Bohr model of the atom. The details of that model are not important here except that it indicates that the overwhelmingly larger part of the atom is nothing but energy and its associated field. In short, the universe, including the most dense of solids is largely empty space. Yet we experience the world around us as largely solid. (Within the model we are using here, liquids can be considered as fluid solids.) The solidity of the universe is an illusion, brought about by interactions of energies and their associated force fields. The really interesting question in physics is about why the universe has any solids at all! Yet regardless of all this we presume to be able to really see, and ignore the fact that what we see is illusory. That is not to say that there is nothing out there, but rather that we cannot know for certain what does lie out there.
Now if all that wasn't enough, our judgements or assessments of the world around us (let's forget the rest of the universe for a moment) is heavily coloured by emotive issues, past experiences, moods and so on. The police and justice systems are aware of these false assessment problems, even if some juries are not. All too often we spend our lives sleep-walking, quite unaware that reality is passing us by. This must be the ultimate in lotus-eating experience, spending our days feeding on illusions.
Ah yes! And I haven't even touched on 'time'!