Nicomachean Ethics

​Hence they seem also to differ in species. For we think that things which are different in species are completed by different things (for this appears to be the case with both natural things and those of art, such as animals, trees, drawings, statues, houses, and vessels); and similarly that activities which differ in species are completed by things that differ in species. Now the activities of the intellect differ from those of the senses, and these themselves differ from one another in species; and so therefore do the pleasures that complete them. This would also become apparent from the fact that each of the pleasures is closely bound up with the activity it completes. For its own proper pleasure increases the activity. Indeed, those who act with pleasure judge each thing better and make it more precise; for example, those who delight in doing geometry become geometers and understand each thing better, and similarly the lovers of music, lovers of building, and each of the others make progress in their proper work because they delight in it. And pleasures increase the activities, and what increases a thing is proper to it; and for things that are different in species, their proper things are also different in species. This would become even more apparent from the fact that pleasures from other sources are hindrances to activities. For lovers of the flute are unable to attend to arguments if they hear someone playing the flute, because they delight more in flute-playing than in the present activity; so the pleasure in flute-playing destroys the activity concerning arguments. This happens similarly in other cases also, when one is active about two things at the same time; for the more pleasant activity drives out the other, and if it differs much in respect of pleasure, all the more, so that one is not even active in the other. Wherefore when we delight intensely in anything we do not do another thing at all, and we do other things when we are only mildly pleased with something, just as in the theatres those who eat sweets do so especially when the competitors are bad. Since, then, proper pleasure makes activities precise, more lasting, and better, while alien pleasures mar them, it is clear that they are far apart. For alien pleasures do almost the same as proper pains; for proper pains destroy activities, as when writing is unpleasant and painful to someone, or calculating is so; for the one does not write, and the other does not calculate, because the activity is painful. Thus, opposite results occur concerning activities from proper pleasures and pains; and proper ones are those that arise upon the activity in itself. And alien pleasures, as has been said, do something very similar to pain; for they destroy, though not in the same way. Now, since activities differ in goodness and badness, and some are to be chosen, others to be avoided, and others neutral, the pleasures are in a similar state; for there is a proper pleasure for each activity.