Fortunately Jonathan explained it for us:
Aristotle, like Hobbes, did think that knowledge came from the senses, but he had a very different view of how senses worked. Aristotle believed that every physical object has a form or essence, and a substance. So a clay model of a tree and real tree share commonalities of form, although their substances are totally different. Aristotle also thought that the psyche is an instrument whereby we can receive the form of objects without the substance. He compares sensation to a signet ring making an impression of wax.Hobbes, however, does not really believe that the concept of "essence" is useful in explaining the world. He is basically a materialist. He believes that the only things worth talking about are matter and its interactions. Therefore, his account of how we obtain knowledge through the senses has to rely on interaction between matter.This might sound like an obscure difference, but it has a lot of consequences for how one studies the world. If you agree with Aristotle, the implication is that by observing the world, you can get an idea of the real essence of things. Acquiring theoretical knowledge is then a matter of thinking rationally about the implications of this knowledge. Thus physical science is a matter of everyday observation followed by rigorous thinking.However, if the information you get from the senses is just a bunch of particles bouncing off of your sensory organs, as Hobbes believes, then there's good reason to be worried that the senses are unreliable, and you need to spend time carefully tweaking the information you get from the senses to make sure you have it right. This gives rise to an experimental model (which Hobbes' contemporary, Francis Bacon, focused on far more than Hobbes did).As for how commonplace it was - Aristotelianism was basically the dominant philosophy from the time of Thomas Aquinas (1200s) up until the 1600s. Hobbes is writing around the time of transition away from Aristotle's position as the preeminent thinker on matters such as this. I actually am not sure how dominant the view still was among academics by the time of the Leviathan.As an aside, the reason Hobbes talks about mediate and immediate interaction is that, at the time, people who subscribed to this materialst view did not believe that matter could interact with other matter at a distance. The only interactions allowed into the theory were direct ones. The view of no interaction at a distance was thrown out after Newton's theory of gravity became the consensus view - since gravity is interaction at a distance.
Hilzoy also stepped in to help us understand the difference:
A few notes: first, when Hobbes talks about the scholastic view of knowledge, ("they say the thing understood sendeth forth an intelligible species", etc.), this is not similar to the way we now understand smell. Scholastics, following Aristotle, thought that objects were composed of matter and form. I.e., a computer is a bunch of metal and plastic and silicon and stuff (the matter), arranged in a particular way (the form). If you had exactly the same matter, but it was a bunch of molten metal and a pile of sand, it would not be the same object; likewise, if you had an identical computer made of different bits of metal and silicon and whatever. 'Species' is scholastic-speak for 'form'; the objects are (according to the scholastics) giving off forms of themselves, whereas (as I understand it) we now think that objects we smell give off matter.Second: I don't recall how significant this is in the rest of Hobbes' work, but the claim that ALL thoughts concern "a representation or appearance of some quality, or other accident of a body without us, which is commonly called an object" is significant, and probably false. Can we think about abstract objects, nonexistent objects, logical arguments, etc.? Are all these "objects without us"? You could argue, as Hume did, that all our thoughts are either about objects of internal or external sense, but can be cut and pasted to create e.g. nonexistent objects. But is it obvious at all that thinking about (for instance) logical validity is in any sense thinking about an object outside us, or is the product of cutting and pasting concepts originally derived from such thoughts? Not to me.
I started this project trying to understand "social contract." I guess we'll get to that eventually, but all the knowledge I'm picking up on the way is awesome. In education we tend to be goal-oriented, and goals are important. But at the same time we forget that part of the beauty of learning lay in all the bits you acquire, almost accidentally, along the way. I think those bits are the stuff of wisdom, as opposed to just "facts."
If I can write you an essay on the history of "social contract" at the end of all this, that's cool. But it would be much better if I could--in some deep way--tell you something about the world of Hobbes, the world of Locke, the world of Rosseau, and what each of those particular worlds means for our own world today. It's one of the reasons why I push us to do something more than study history as a way of refuting our racist uncle at Thanksgiving, or serving the homophobe from high school who keeps popping up on our Facebook feed.
If you spend your day debating whether the earth is flat or not, how do you ever get to the truly profound questions of cosmology? It's not within your power to banish ignorance from the world, or even from Facebook. Besides, you have your own ignorance to bear.
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