Aristotle does not go so far as to say that pleasure is the ultimate good, but he comes as close to it as one can get. The claim, however, comes with Aristotle’s striking and radically new account of what pleasure is, which argues, inter alia, that physical pleasures are not pleasures simply speaking and that nothing that comes with a vicious or degraded action can be called pleasant at all.
Aristotle’s definition of pleasure seems drawn from two principles: 1.) pleasure is something one is aware of, and so belongs to things that have knowledge (whether of sense, reason, etc.); and – a premise that follows from this 2.) pleasure should be understood by considering the way an activity like sight or knowledge is complete. In developing (2) Aristotle compares being pleased to actions building or walking and notes that pleasure is is whole and complete from the moment the action begins while building is not. After a few paragraphs of examples, Aristotle claims that pleasure is simply not a motion at all, and it therefore is not in time. Time is essential to some X only if the being of X requires development or change, but pleasure of itself is defined in opposition to this. Aristotle pushes this opposition further by claiming that changeable existence is the source of inconstancy, absence, and loss of pleasure.
It follows from this that the standard or measure of pleasure is the eternal knowledge and activity of a supremely and eternally existent being. The search for pleasure even in finite beings suggests a theistic proof so far as it admits of no perfect explanation except when seen as a participation in the divine life.