For philosopher John Locke, knowledge about external world comes from experience. Our senses play a major role in perceiving and understanding objects and transforming them into ideas we have about external world. For myself the proof of existence of external world comes from sensation. If I can see, hear, touch, smell, taste
and feel objects, than my perception of external world is real.
The perception means the power to know something. When this power reaches an object, it moves to actuality. In other words when perception takes place, we come to know what the object of perception is and from knowing a thing we come to awareness. When we perceive we exist and the world around us becomes reality. The world seems objective. A chair, a tree, a glass of water all seems to be physical things that we can feel. Perceptual information and cognitive information are both unified into construction of reality.
Note, I am not saying that external world in real Universe does actually exist the way I perceive it, I am simply saying since I can sense and through perception come to awareness about the external world than in my mind it becomes reality. I agree that there's nothing in the world that makes a table a real table. The concept of "table" is something that's constructed by our minds. But I don't see how that indicates there's no external reality. Maybe there's just a collection of matter out there in a certain arrangement; we perceive the matter, and our minds organize them into concepts (table, chair), which allow us to understand them. You can deny that "tables" are externally real without denying that the external world exists, can't you? So, if I can't perceive an
external world for I am not the external...