Although an increased attention has been given to the doctrine of the association of ideas as being sufficient to account for most of the operations of mind, yet its nature has been looked upon as too simple and philosophical to require much scrutiny; whereas, that very power of association appears to me the most difficult of comprehension in nature; for how shall any given idea be supposed as associated with some other idea, which idea is not yet supposed to be in existence; one idea only present in the mind, a single simple perception, merely, cannot suggest an after perception, for the suggestion is the perception of the suggested idea itself.
Lady Mary Shepherd, Essays on the Perception of an External Universe, Essay XIII.
This brief comment seems to me to be of extraordinary importance, far more than it might seem at first glance. It raises a problem that early modern empiricism never answered satisfactorily and early modern rationalism only answered (as in Malebranche) with claims that did not take hold and are often considered quite extreme; it is one of the key problems of semiotics; at least a modified form of it is a significant problem for naturalistic epistemologies, since it is closely related to the question of the limits of human cognition. You do not have to be rigorously associationist to be faced with the problem or some variant of it. It is, in short, one of the major problems of modern times and, I would say, far more important than many of the problems in epistemology that get greater attention. Shepherd's own suggestion is that association is already the union of different things under one concept, and thus necessarily presupposes more fundamental acts of mind; this is part of her ongoing assault against the empiricisms of Berkeley and Hume.