Central question
When Berkeley roundly dismisses the existence of 'abstract' ideas and pronounces that 'to be is to be perceived', is he implying anything more than that the world is an illusion?
I don't think Berkeley believes or implies that 'the world is an illusion'. It is perfectly real - only it is not real as a material object, or set of material objects, that exist without being perceived. The following appears to be a plausible account of what the doctrine of abtract ideas - i.e., the doctrine that there are no abstract ideas - is taken to imply:
BERKELEY was convinced that the doctrine of abstract ideas lies at the base of many philosophical errors in metaphysics, morality, and
speculative science. In particular, he suggests, the "strangely prevailing"
opinion that sensible objects "have an existence.. .distinct from their being
perceived" (Principles, §4) rests on the doctrine of abstraction. Berkeley
regards the belief that the color and shape of a rose can be separated by
abstraction from the rose itself as the first step towards the belief that the
rose can exist unperceived. Abstraction is also responsible for the illusion
that morality is an elusive subject (Principles, §100). Moralists falsely
assume that we must have precise abstract ideas of justice and virtue to
theorize about ethics. Finally, Berkeley believes that the two great
branches of speculative science - natural philosophy and mathematics -
are also infected by abstract ideas. Physicists believe in pure space because
they think that through abstraction they can create an idea of space
distinct from any body, or motion (Principles, §116); geometers believe in
infinite divisibility because they think lines are abstract ideas, and hence
they can have properties no sensible line has (Principles, §125).
(Zoltán Szabó, 'Zoltán Szabó', History of Philosophy Quarterly, Vol. 12, No. 1 (Jan., 1995), pp. 41-63: 41.)
It is an open question whether 'his [Berkeley's] attempt to bring anti-abstractionism into harmony with immaterialism ultimately fails' (Szabó: 41); and Szabó argues at length and in detail that it does fail. That, however, can be bracketed out for the present purpose since the question at issue is what Berkeley implies by hus rejection of the existence of abstract ideas. Pretty clearly, or so it seems to me, he does not imply that the world is an illusion.
Terminological note on 'the fine and subtle net of abstract ideas' - Principles, Intro, XII)
It is by no means clear that Berkeley uses the notion of abstract ideas uneqivocally. Three senes may be noted:
Abstract ideas - (1) the single property view
This involves saying that it is
possible for there to be an abstract idea, which is an idea of one
quality only, even though that quality cannot be instantiated
unless others are instantiated with it. We find this set out in §7, For example, there is perceived by sight an object extended, coloured,
and moved: this mixed or compound idea the mind resolving into its
simple, constituent parts, and viewing each by it self, exclusive of the
rest, does form the abstract ideas of extension, colour, and motion.
Not that it is possible for colour or motion to exist without extension:
but only that the mind can frame to it self by abstraction the idea of
colour exclusive of extension, and of motion exclusive of both colour
and extension. (E.J. Craig, 'Berkeley's Attack on Abstract Ideas', The Philosophical Review, Vol. 77, No. 4 (Oct., 1968), pp. 425-437: 425-6.)
Abstract ideas - (2) the common properties view
In this the abstract idea is
compounded of ideas which are instantiated by every instance of
the concept in question. Consider it as it appears in w. In this the abstract idea is
compounded of ideas which are instantiated by every instance of
the concept in question. Consider it as it appears in w. In this the abstract idea is
compounded of ideas which are instantiated by every instance of
the concept in question. Consider it as it appears in §9, where
we have:
For example, the mind having observed that Peter, James, and John,
resemble each other, in certain common agreements of shape and
other qualities, leaves out of the complex or compounded idea it has
of Peter, James, and any other particular man, that which is peculiar
to each, retaining only what is common to all; and so makes an
abstract idea wherein all the particulars equally partake, abstracting
entirely from and cutting off all those circumstances and differences,
which might determine it to any particular existence.
It is logically possible to hold this view of abstract ideas and dismiss abstract ideas in the previous sense:
For let us suppose that there is a property A, such that
possession of A entails possession of some other property B.
Then any objects which have A in common will also have B in
common. Consequently, in abstracting common properties, it is
open to us to abstract B as well as A, and nothing about (2) as stated commits one to saying that it is possible to abstract A
alone, without the necessarily co-extensive B. (2), of course, does
not require that an abstract idea should include ideas of every
property common to all the instances. But it does not forbid this;
a fortiori, it never forbids the inclusion (with the idea of A) of
ideas of any other properties which necessarily accompany A.
The common properties view does not commit one to the existence
of any Type i abstract ideas. It is important to realize this, since
one might otherwise think that Berkeley's objection to (i) also
constitutes an objection to (2).
(Craig: 426-7.)
Abstract ideas (3) - the full representation view
This comes in §3, where
Berkeley quotes the celebrated passage from Locke's Essay, IV,
vii, 9.
The abstract idea contains, it seems, ideas of all the prop-
erties of all the instances of the general term in question:
the description that is here given of the general idea of a triangle,
which is, neither oblique, nor rectangle, equilateral, equicrural, nor scalenon,
but all and none of these at once [original italics; §133, end].
There are triangles which are rectangular (and hence not equilateral) and there are triangles which are equilateral (and hence
not rectangular); consequently, the general idea must be an idea
of a triangle which is both rectangular and not rectangular,
equilateral and not equilateral, and so on.
It is obvious that (3) differs sharply from (i) and (2), but this
does not emerge from Berkeley's presentation. He prefaces his
quotation from Locke with the words:
To give the reader a yet clearer view of the nature of abstract ideas .
I shall add one more passage from the Essay on Human Under-
standing, ....
This way of putting it surely implies that he is telling us more
about abstract ideas of the kind already described and discussed.
This is mistaken. That (3) is not the same as either (i) or (2) can
be seen from the simple consideration that the latter ideas have
no constituents but what are common to all the relevant particulars,
whereas the ideas of (3) include components possessed by only
some of the instances.
(Craig: 429.)
If we immerse ourselves in the relations of (1), (2) and (3) we will never
resurface - at least not here. But we should be aware of the interpretative problem if only to check the sense of 'abstract idea' that in our considered judgment applies in particular passages.
