Akroma, he didn't invent the scientific method, he laid the ground work for pre-cursors. He was the first major philosopher to even really suggest that we could gain knowledge through the use of empirical knowledge. However, he did not check whether his claims were right or not, because in an amusing twist, he believed that you could simply think about such issues long enough and come up with a suitable answer using logic. He didn't ascribe to the experimentation that is now an integral part of the scientific method, but he was still a philosophical pioneer.
Sure, it would have been great if he actually sat down and spent time with deaf people, but at the time, nobody did, but most people had unsavoury ideas about the intelligence of deaf and dumb people. He wrote prolifically, and the fact that his writings must represent an intolerant cultural zeitgeist is unfortunate because I don't think he deserves the blame. "Random" monks were better placed to teach deaf people because they had already a certain sign language in place to circumvent vows of silence. Aristotle was not even a pseudo-scientist because science didn't exist at the time.
hellalt, fair enough, but you said "speech", not "language", which makes a very different case. Deaf people do have language. Your statement sent alarm bells ringing in my head because it's believed that hominids first communicated via sign language, a vestige that is still seen through hand gestures which supplement speech, as well as other non-verbal manners of expressing information, such as through facial expression and stance. The article you linked to only claims that language may shape thought, but actually admits that the strong view of linguistic determinism has long been abandoned. "This strong Whorfian view—that thought and action are entirely determined by language—has long been abandoned in the field."