He concludes that a candidate must have capacity for speech, as in have the proper organs for speaking AND the capacity for rational activity. However, the candidate must be able to have a rational, relevant conversation and not just an expression of passion. .
Descartes describes passion as a reaction to a certain feeling that does not fully require thought and in turn decides that the capacity for linguistic activity, as described above, are the only "external actions" that are able to show a thinking mind. Some examples of actions based on passions given by Descartes include eating, walking and protecting oneself from falling down. These actions, Descartes believes, are merely our reactions to feelings that in fact do require some thinking in our part but that would happen nonetheless (CR, p. 36). In other words, these actions of passion, eating or walking, are things one do subconsciously, it requires some thinking but not as much as having a reasonable, intelligent conversation does. What ultimately differentiates humans from animals according to Descartes is innate possibility (CR, p. 36). Thinking complex enough to have a meaningful conversation with someone is something that is a matter of learning maturing as individuals rather than learning or being born with that trait. Eating is something that is innate for both humans and animals, when something is placed in their mouth their first instinct (action of passion) is to chew; the only difference is that humans must mature enough to be able to hold a spoon, not only learning from a parent but being able to physically hold a spoon and remember to do it each time. The difference in innate tendencies, actions of passion and the capacity of language are what separates animals and humans and in turn causes some creatures to be excluded.
With this definition Descartes manages to exclude animals but include humans that have some sort of mental capacity and sometimes would be thought of as not having linguistic capacity.
