I look through my window and acquire this information. There is a tree outside. I have just increased what many philosophers would describe as my knowledge of the external world. When I consider what I have just come to know, the following appears evident. The existence of a tree outside my window does not, in any philosophically interesting sense, depend on my propensity to believe in its existence. It does not even depend on a collective propensity to believe in its existence. In this sense I have become informed about a state of affairs that obtains independently of whatever I, or anyone else, think about it.

Arguably, the situation is different with colours. The tree outside the window is brown. The fact that there is a tree outside my window is not in any way constituted by my believing that there is one there. In contrast, Crispin Wright argues that the fact that the tree is brown is constituted by an individual believing, in suitable circumstances, that it is a fact.

Of course, this is one way to capture the thought that colour is a secondary property. Standardly that thought is articulated in something like the following way. An object having a certain colour amounts to no more than its having the propensity to induce in normal observers, in the right circumstances, a characteristic sensation. In consequence, it is sometimes said, the concept of colour is response dependent. Something falls under a response-dependent concept in virtue of eliciting an appropriate response in an appropriately situated observer.