\subsection{Descartes’ Three Grades of Sensory Response} ‘when I see a stick, it should not be supposed that certain ‘intentional forms’ fly off the stick towards the eye, but simply that rays of light are reflected off the stick and set up certain movements in the optic nerve and, via the optic nerve, in the brain, as I have explained at some length in the Optics. This movement in the brain, which is common to us and the brutes, is the \textbf{first grade of sensory response}. This leads to \textbf{the second grade}, which extends to the mere perception of the colour and light reflected from the stick; it arises from the fact that the mind is so intimately conjoined with the body that it is affected by the movements which occur in it. \emph{Nothing more than this should be referred to the sensory faculty, if we wish to distinguish it carefully from the intellect.} But suppose that, as a result of being affected by this sensation of colour, I judge that a stick, located outside me, is coloured; and suppose that on the basis of the extension of the colour and its boundaries together with its position in relation to the parts of the brain, I make a rational calculation about the size, shape and distance of the stick: although such reasoning is commonly assigned to the senses (which is why I have here referred it to \textbf{the third grade of sensory response}), it is clear that it depends solely on the intellect’ \citep[p.~295, AT VII:437]{descartes:1985_csm2}.