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So we can set up situations in which forces are acting and not acting, while diffusion occurs anyway. This shows those forces do not cause diffusion.

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Yes correct. Good observation. And as you mentioned about the question - sometimes the end result of an investigation leads us back to the question again. We come up with a better way of asking the question. If you want to ask "What causes diffusion" it's the Second Law of Thermodynamics as I mentioned earlier and as you confirmed in the online encyclopedia.

In physics problems sometimes most of the problem is figuring out where to start in terms of describing it. It can be a pain in the butt to know where to start. Some problems are best described in terms of forces. Others classes of problems are best described in terms of the law of conservation of momentum for example.

This problem that you have stated is best described by the Second Law of Thermodynamics which states that in a closed system there is a tendancy over time to move from order to disorder. That's all there is to this really. As the particles bang around they distribute themselves randomly throughout the room. Its the statistical tendancy of things that start out ordered to move to a disordered state that really "causes" diffusion of the perfume in the case that you have given. The Second law of Thermodynamics is the starting point for explaining a great many things. There are a whole categories of problems which draw on this Law as the "cause" behind the problem. I used to do short reviews for the MCAT (test for med school) and the MCAT test questions often have the second law of thermodynamics as correct choice as to the reason why something is happening. It fits into biological processes a lot.

Best of luck