Hi all! I was directed (on another thread) to professor Leonard Susskind's first lecture on classical mechanics. I learned a lot from it, however he introduced a "rule" for "acceptable" classical fields/equations that I have a little trouble with. He explains classical mechanics as if you know the position in space and time of an object, you can determine everywhere it has been, and everywhere it will go (I know my wording is a little off). He says that if you have a flowchart of the states of the particle, each state (in order to be "acceptable" in classical mechanics) may have no more than 1 arrow going to the state, and 1 arrow going away from the state. This, as I have seen it, generally works, however I believe I have found an exception that I don't think he talks about. In a field with a point that acts as a sink, the sequence may look like this (with the sink being r0): r3->r2->r1->r0->r0 and so on. If you made a flowchart, the state r0 would have 2 arrows pointing towards it (see the lecture if you are confused with what I'm saying). Many fields with sinks, such as the flow of a fluid or gravity, are in classical mechanics, but they break this rule. Am I understanding this correct? Thanks in advance!