For those interested in a little more rigor, there's a great series of lectures on the subject that was just posted online from Steve Awodey, who wrote one of t

For those interested in a little more rigor, there's a great series of lectures on the subject that was just posted online from Steve Awodey, who wrote one of the best introductory texts on the subject. https://www.youtube.com/watch?v=BF6kHD1DAeU https://www.youtube.com/watch?v=BF6kHD1DAeU