Berkeley’s Three Dialogues between Hylas and Philonus George Berkeley was a proponent of what came to be known as subjective idealism, a theory that denied the existence of material substance, contending that such things could only exist in the mind.

 The dialogues are fictional exchanges between Philonus, the subjective idealist, and Hylas, his friend who holds to the existence of matter.

Having read the dialogues, I found myself completely unconvinced. But I wanted to understand how Berkeley constructed his argument- and what he did to skewer the argument in his favor. So I went back to the first dialogue and paid particular attention to the form.

 The basic premises are as follows. Philonas argues there is no such thing as material substance. Material substance is defined as that which can be perceived by the senses. A material substance, in order to be perceivable, has sensible qualities. If one were to remove those qualities, then there would remain nothing to be sensed, and therefore, sensible things are nothing but so many sensible qualities.

 Hylas notes there is a difference between existence and perception.

 Philonas offers the example of heat. If heat is something real, it must exist without the mind. This would include heat’s existence to all degrees. The most intense degrees of heat bring great pain. And something that can’t sense is incapable of sensing pain. He then asks if the material substance is senseless? Or sensible?

If it is incapable of sense, it is incapable of pain, and therefore it can’t have the property of heat, since great heat would produce pain and it is incapable of sensing pain.

 Hylas correctly observes that pain is distinct from heat.

 Philonas responds with the observation that if you put your hand closer to the fire, you sense both great heat and pain, so they are indistinct. Therefore, great heat can’t exist apart from a mind perceiving it.

 There are two misdirects I see in Philonus arguments. The first is when he tries to tie sense with the sensible qualities of heat. Granting that heat has sensible qualities, it does not follow that heat only has effects on a material substance that can sense.

Berkeley understands this by having the hypothetical friend observe that pain is distinct from heat.

But Berkeley unfairly narrows the scope of the material substance in question to those that can sense.

 It’s true that for a creature that can sense, high heat will be accompanied by pain. But pain is distinct from heat. Heat will have an effect on a wood structure even though the structure doesn’t feel it. The sensible qualities don’t have to be sensed to be real, which we can see when a fire burns a house down. A man can stand a mile off and watch a house burn down without feeling pain.

 So the trick is to convince your opponent that the scope of the argument must necessarily be narrowed to the scope that achieves what you want. At the point he buys in and accepts it as true, then it is conflated out to cover a scope where it doesn’t work. It is an ancient trick: it’s true, but it’s not the whole truth.