I had a mentor of sorts who was this retired engineer who went to MIT and he gave me the advice that bonsai is one half art, one half biology, one half philosophy. This adds up to over 100% obviously but the point was that to be good at bonsai you needed to have at least two of those things down pat. The biology is interesting, (just kind of stream of thought here) although I was pretty young at the time I remember there being a great deal of interest in soil mixes, and special types of fertilizer which allow the tree to be healthy while also staying small. I was also taught to think of the root structure as a mirror of the visible part of the tree, and in some ways more important than the parts that you typically see. You can recover a tree from pruning its branches but if you over prune the roots it will likely die, similarly you can't let the roots overgrow because they will get over crowded in the small pots. Also certain species of tree with different root structures required differently shaped pots. Grafts were common, sometimes you might desire a tree that does not grow well in pots but you could achieve the same thing by grafting a branch of one species onto the root stock of another but this was difficult to do because you can end up with ugly scars or a contrast between the two types of bark. Another thing I remember being interesting is that some fruiting trees will produce miniature fruits, or miniature flowers.