Principles of Human Knowledge summary

Let’s go back to that apple. When you survey your knowledge of it, what do you actually find? You find a certain color, a certain smell, a certain hardness. These are all what Berkeley calls ideas.

Think about it: can a color exist without being seen? Can a sound exist without being heard? Can a pain exist without being felt? To Berkeley, the answer is a resounding no. An unperceived color is a contradiction in terms. So since the apple is just a collection of these perceived qualities, the apple itself can’t exist without a mind to perceive it.

In Berkeley’s day, many scientists and philosophers tried to compromise. They admitted that secondary qualities like color, taste, and smell might just be in the mind. But they insisted that primary qualities – like size, shape, and motion – really existed in material objects.

They argued that while the sweetness of a sugar cube might be a subjective sensation, its squareness was an objective fact of matter.

If the world is just ideas in our minds, a scary question arises: Does the tree in the forest disappear when no one is there to see it? Do your furniture and your house vanish the moment you close your eyes or go to sleep?

For a skeptic, this leads to a nightmare of solipsism – the belief that only your mind exists. But Berkeley was no solipsist. He pointed out a crucial fact: we don’t choose most of our ideas.

One of the most surprising claims Berkeley makes is that his immaterialism actually defeats the arguments of the Skeptics, and rescues common sense.

Skeptics of his time argued that because our senses only show us images of things, we can never be sure if there is a real world behind those images. We see a red image, but for all we know, the real object is a gray blob or doesn’t exist at all. In this view, we can never truly know reality; we only know a mental copy of it. We’re like people looking at a movie screen, wondering if there’s actually a world outside the theater.

At this point, you might be thinking: Fine, Berkeley. But if I walk into a wall, it’s still going to hurt. Does calling it an “Idea” change anything?

Berkeley would agree with you. He wasn't suggesting we change how we live. He famously said we should “think with the learned, and speak with the vulgar.” We can still say the sun rises and the fire is hot, even if we know that scientifically and philosophically, those are just descriptions of shifting ideas.