In discussing how perception is caused by the object of perception, Aristotle takes as an example the nature of food and how it feeds us.
Not everything is food. Only that which feeds can really be called food.
Likewise, not everything in existence can become objects of perception. Only that which is capable of becoming objects of perception can be called objects of perception.
This is true also of our minds and thoughts. Only that which is capable of becoming objects of thought can truly be a thought.
Why is this important? It is so because it reveals to us that we have a limited awareness of reality. Our mental and perceptual capacities only reveals certain structures of reality.
For example, we may imagine that an alien race has other faculties that allows them to understand something about reality that we may not have access too.
This is also a problem when we try to describe the nature of God. How are we supposed to know what capacities God have? Certainly they will be a lot different from ours.
As another thought experiment, we might consider that it would be possible to develop other senses that them we currently have. We might imagine an object which is not capable of being registrered by our current senses.
This object might not reveal itself to any of our senses such as our sight or ears. This would mean that it would be there, yet we would be unable to sense its presence.
This is not entirely irrational. We already know that there are sounds that we cannot hear except through instruments and likewise with certain forms of light. So we might imagine object that can only be revealed through a sense that we have not developed.