 Try to think of color without attaching it to an item.  A concept such as color may only exist in our realm as it is attached to another object.  A purple flower, a purple dress, a purple book.  Without the recognition of these objects as being existent, the color attached to them would not exist.

 We assert that purple exists simply through its attachment through a tangible object, and the a priori applies as a recognized fact (that the flower, dress, or book is an existent item), but,  When does purple cease to be purple and become blue, black or pink? The concept is a personal/subjective one.  If a colour is a subjective perception that we recognize as existent, then so to…

 If humans are tangible objects, then the thoughts and perceptions that humans hold and attach themselves to must also be existent.  Descartes’ cogito: “I think, therefore I am” can be extended to mean: I think, therefore I am and my ideas, concepts and perceptions that exist within my thoughts are attached to me, (the tangible being) then my thoughts are as valid a notion as Purple.

 If we assert that the rationalist theories/philosophies of Descartes make sense, then we recognize that  People are tangible, and therefore exist. If they, themselves exist, so, too must their thoughts. If they believe that: God exists, people are kind, futures are predetermined…, then these concepts/ideas are very real; as real as colour on an autumn day.