Most fortunately it happens, that since reason is incapable of dispelling these clouds, nature herself suffices to that purpose, and cures me of this philosophical melancholy and delirium, either by relaxing this bent of mind, or by some avocation, and lively impressions of my senses, which obliterate all these chimeras. I dine, I play a game of back-gammon, I converse, and am merry with my friends. (Hume, 1737, 1.4.7, p. 175)
The passions, according to Hume, as with Descartes, are intense emotional propensities. They are not, as with Spinoza, the negative aspect of the affects. What motivates someone to act are not his reflections but his passions. The passions are therefore that part of the mind that impulsively adopts an option in a sufficiently intense and durable manner to create a behavior. Passions want to attain a goal associated with an object, and they have difficulty integrating the results of reflection into their project. Passions prefer to ally themselves to other passions that augment their intensity than to rational thoughts that often put a brake on a propensity to act by reducing their intensity and the pleasure associated to this intensity.12 The pleasure that is associated with the intensity of the passions is independent of their content. Therefore, there would be pleasure in intense hate.
To designate this stubbornness of intense ideas, Hume sometimes uses the terms propension or propensity, as in the quote at the beginning of this section. I use the term propensity to designate, in a broad fashion, all the forces of an organism that repeatedly aim at the attainment of a goal. A propensity is an automatic tendency. These automatic tendencies coordinate the heteroclite mechanisms of the organism Hume perceives the mind as a republic within which diverse entities are united by reciprocal subordinate relationships to a government. They propagate themselves in this republic by unceasingly exchanging their riches.13 This metaphor is close to the one proposed in 1985 by Marvin Minsky in The Society of Mind. This was one of the first works that made the models of artificial intelligence accessible to a large public. It is possible to distinguish two types of propensities:
1. Internal propensities coordinate the heteroclite mechanisms situated within an organism (ideas, impressions, body movements, etc.) to accomplish a task.
2. External propensities relate social rituals to internal propensities. Thus, an urge to eat stimulated by a commercial is an example of an external propensity.