	That everyone (at least everyone without neurochemical deficiency) wants to find food upon feeling the physical sensations of hunger, or wants to find water upon feeling the physical sensations of thirst, is unquestionable. The question of whether these associations are actually instinctive depends on whether or not we are born with them or whether or not we learn them. Being born with these associations would manifest itself by the fact that the physical structure of a newborn's brain already has all the connections needed which would cause it to perform certain behaviors upon the presence of a certain sensation - for example crying when it is hungry. If the associations are not instinctive, this would mean that a child is born with no idea what the tugging sensation in its stomach means and somehow figures it out. 
A proposition in between these two is that the child is born with the foundations for developing the sensation between these two ideas, but must have experience to learn them. For example, a child may be born with direct neural connections that cause its mouth to open and its vocal chords to vibrate as a direct result of chemical reactions in its empty stomach. The child would not need to have any idea of what this means. It could be a simple physical reaction that the child plays no conscious part in, something like the way the knee jerks when tapped. Just this much instinct could be a foundation for a later, more sophisticated association between the ideas. As it grows, and learns that whenever it cries it gets food, it may begin to consciously "decide" to cry when it is hungry, in order to get food, which may further develop into the adult associations that Descartes speaks of. It is difficult to say whether this is how it actually works or to think of a way to figure out if this is how it actually works, because nobody really remembers that far back. It is a matter of choice to decide whether to call this type of learning-with-help-of-innate-foundation instinctive behavior or not.
