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Ryan asked:

Can Realism be defended by claiming that it offers a better explanation than anti-realism about why
we can affect our surroundings? It's very easy in physical terms to explain why I can lift or push an
object, but if things only existed in perception, it seems that there would be no real account for this.

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The Berkeleian idealist or 'immaterialist' denies the existence of objects external to the mind.
However, your question also concerns a form of opposition to realism which arguably goes back to
the Ancient Greek sophist Protagoras:

Of all things a measure is man — of the things that are, that they are; of the things that are
not, that they are not.
Jonathan Barnes The Presocratic Philosophers§491, p. 541

On the Protagorean, or 'anti-realist' view of truth, there being things that are the casedepends on the
possibility of human knowledge. There is no place in the Protagorean universe for 'facts' or 'truths'
which human beings are incapable of knowing to be the case. In recent times, Michael Dummett, has
revived this form of anti-realism, an issue which I myself have taken a keen interest in. (See Answer
to Bill, Answer to Joanna, Answer to Rute and Alan, Answer to Glouisel.) In what follows, I shall be
talking about my interpretation of anti-realism, rather than Dummett's.

IdealismYou say that 'it is very easy in physical terms to explain why I can lift or push an object'.
What you are talking about is the physical causation involved in the movement of your hand followed
by the movement of the object. Berkeley has two replies to this, however:

(a) If we are concerned merely with the lawlikebehaviour of objects which we observe in the world
around us, then Berkeley would say that it is just as easy to conceive of those laws applying to the
world of objects inside God's mind as it is to conceive of them applying to the realist's world of
'material' objects. In this respect, realism and idealism are on a par. The laws of nature are equally
'real' whether we conceive of them applying to the realist's world of matter in space, or to the idealist's
'virtual reality' world of mental 'ideas'.

(b) Berkeley anticipated Hume in questioning whether from the mere appearance of the movement of
one object, followed by the movement of another object we are able to derive any meaningful notion
of causation. Berkeley believed that that it is only through acts of will that we actually perceive
causation in action. So he would deny your claim that it is easy to explain how physical change takes
place. It is much easier, he would argue, to grasp the real workings of causation in the world of 'ideas'
than it is to grasp it in the realist's world of dumb 'matter'.

Anti-realismA version of your objection can be formulated in the context of the anti-realist view of
truth. Think of all the things that mightbe the case, consistently with all the things we could ever know
to be the case. According to the anti-realist, each of these alternatives describes a possible world,
none of which carries any special metaphysical 'marker' to designate it as the actual world.

Let us suppose a historian comes across a manuscript which they have good reason to believe from
the context, date and stylistic considerations was either the work of author A, or the work of author B.
Suppose also that all evidence of whether it was A or B who wrote the work has been destroyed.
According to the anti-realist, the substance our world, the world which we call 'actual', is made up of
possible worlds, including the possible world in which A was the author and the possible world where
B was the author. Both possible worlds are equally 'real'. But how can a determinate object, such as a
manuscript which I actually hold in my hands, have an indeterminate cause?

This objection effectively destroys the idea that our actual world is determinate so far as we are able
to uncover facts about it, and only indeterminate round the edges, beyond the limits of possible
human inquiry. However, this does not have to be the anti-realist view. The alternative is to deny that
there is such a thing as themanuscript, or thehistorian who holds it in their hands. 'The' manuscript is
in reality the work of A, and at the same time isin reality the work of B (and of C, and D and all the
other possible individuals who might have been its author). In other words, according to the
anti-realist, the truth about our world is indeterminate all the way to the core.

Geoffrey Klempner