It is quite true that 'the simple and illiterate never pretend to abstract notions,' for the sufficient reason that they do not know the names of their mental operations, even if they are capable of discriminating them. For the same reason they do not pretend to talk prose or to be realists.
The practice of every profession and craft, even the humblest, involves abstraction and generalisation. The objective properties associated with a given utility have to be abstracted from those which are indifferent, and this is what enables men of experience in any branch of industry or art to form a speedy judgment on matters touching their special affairs. It is in part what distinguishes the 'professional' from the 'amateur.'
Berkeley's disclaimer of any power in himself to form general ideas is no doubt sincere, and he is justified in reasoning from himself to others. But the point at issue is, whether Berkeley in this instance correctly analysed his own mental processes. The fact that he was correct in some points of great importance does not preclude us from surmising that he may have been wrong in others of less importance. In comparison with his discovery of the substantiality of mind, his oversight on the subject of abstraction is a bagatelle.
He explains the existence of general words on the theory that they are names of particular ideas which we use to represent all similar ideas.
'... an idea which, considered in itself, is particular, becomes general by being made to represent or stand for all other particular ideas of the same sort. To make this plain by an example, suppose a geometrician is demonstrating the method of cutting a line in two equal parts. He draws, for instance, a black line of one inch in length: this, which in itself is a particular line, is nevertheless with regard to its signification general, since, as it is there used, it represents all particular lines whatsoever; so that what is demonstrated of it is demonstrated of all lines, or, in other words, of a line in general. And, as that particular line becomes general by being made a sign, so the name "line," which taken absolutely is particular, by being a sign is made general. And as the former owes its generality not to its being the sign of an abstract or general line, but of all particular right lines that may possibly exist, so the latter must be thought to derive its generality from the same cause, namely, the various particular lines which it indifferently denotes.'
These extracts will suffice to show what was Berkeley's doctrine on the subject of general ideas.
With respect to the analogy supposed to exist between the generality of a name and the generality of a general idea, it has to be observed that a name owes its generality solely to its being the sign of a general idea. It is an imputed or conventional generality,—in its proper character a general name is concrete and individual. Also it does not resemble the thing it signifies (the general idea), nor the concrete things from which that has been derived.
The generality of a general idea, on the other hand, depends altogether on its resemblance to many particular things. It is independent of convention. Hence there is no real analogy between the two generalities.
Considering that Berkeley professes himself unable to imagine abstract properties, it is surprising how easily and naturally he writes about geometrical lines—which are abstract properties. Probably he means concrete strokes.