He distinguishes luck from mere chance.
Luck applies only to human actions whereas chance applies to any natural event.
Each is contrasted with what happens from some human or natural cause. Fortune or luck is what happens beyond the intended effect of my actions, for instance, if I should go for a walk to get some fresh air and find a hundred-dollar note. That is good luck. If I should trip over and hurt myself, that is bad luck. A chance event is one that happens outside the usual course of things, i.