Association and Ground -- "We must say that just as one way in which a mental event causes a subseqeunt mental event is by Association (when I think of parsnips I think of my first school), so another way in which it can causes it, is siply by being a gound for it. ..." One kind of linkage is occasional and so not really caused by but occasioned by, and the other is an actual cause. One kind of linkage is more intimate than the other, but one kind is more associated with thought. Lewis' point is that if thought is all cause and effect then it isn't thought at all.