Porn, on screen or on paper, is a great, though rarely used, example for causal interaction between mind and body. Wait, you will say: between mind and body? Isn’t that presupposing the very distinction monists want to question? Well, no. Distinguishing mind and body here is merely expository, meant to capture the following difference: when presented with a haptic stimulus, no representation of the object (understood broadly to include human beings) is needed to produce a particular reaction. Think tickling, if you prefer to think innocently. This is not true for visual stimuli – there you have to represent the object as something in order to have a certain reaction to it. Again, if you prefer a less controversial case, think about comedy, even fairly simple minded comedy.
Mind, in this context, then just means: capacity to represent objects as objects; mental states (as opposed to other states) are capable of such a representation. This does not in itself preclude the possibility that any such mental state is also, in some way or other, a brain state (or some other physical state). It is merely an attempt to get at a difference that seems to be present in certain phenomena, of which watching porn is an example.
Porn produces a certain bodily reaction (it’s about as bodily as reactions go) through visual stimuli (let’s stick to the easy cases and leave aside erotic novels). In order to produce the reaction, a representation of the object is needed, so here it seems we have an interesting case of mind-body interaction. It is because you represent what’s going on on screen as a particular kind of action, that you respond to it in a certain way. The reaction may be inevitable given the representation, which may tempt us to liken it to cases of haptic stimulation. But the fact that it would be very difficult to try to represent what’s going on as something other than what it evidently is (this really is just what it looks like!), this doesn’t mean that you don’t have to represent it as such.
In watching porn, we volunteer to subject ourselves to a particular kind of manipulation: we know ahead of time, that we will respond to what we see in a particular way. We may even think that we cannot help but respond in that way. It’s a bit odd to think that we seek out such manipulation, no?