WTH are knives being talked about?
Knives are not illegal. (the ones that are are meant to kill) Murder is illegal. In most cases you can easily possess a knife and not get a fine.
vs.
Pot is illegal. You don't have to do anything with it for it to get you into trouble. In most cases you cannot posses any form of marijuana or else you get a fine.
The knife argument, no matter what for, is a terrible counter-example. Man manipulates the metal found in a factory. You plant a seed and the illegal substance grows with little to no help at all. You cannot plant a knife and have it grow to be illegal.....??? A knife is not natural unless it is carved from wood, in which case, it is not illegal.