Summary: A Treatise Concerning the Principles of Human Knowledge (page 4)

Finally, it is impossible to determine whether external material bodies exist, because either they can be perceived—in which case they are ideas—or they cannot be perceived, in which case their existence cannot be proved.

It is impossible to conceive of a sound, a shape, or a movement existing outside the mind or unperceived. We can, of course, imagine trees in a park with no one nearby to perceive them. But do not you your self perceive or think of them all the while? This therefore is nothing to the purpose ¹. In fact, to make out this, it is necessary that you conceive them existing unconceived or unthought of, which is a manifest Repugnancy ².

Berkeley asserts a fundamental property of ideas: they are inactive, passive, and inert: It is impossible for an Idea to do any thing, or, strictly speaking, to be the Cause of any thing ³.

There must therefore be some Cause of these Ideas whereon they depend, and which produces and changes them ⁴. If this cannot be an idea (which is passive), nor a material or corporeal substance (which does not exist), then it must be an incorporeal active Substance or Spirit ⁵.

Berkeley defines it as follows:

A Spirit is one simple, undivided, active Being: as it perceives Ideas, it is called the Understanding, and as it produces or otherwise operates about them, it is called the Will ⁶.

We cannot form an idea of a soul or intelligence, for our passive ideas cannot represent to us that which acts. Yet if we cannot perceive it in itself, we can perceive it through the effects it produces.

Berkeley notes that while I can act on some ideas (for example, imagining a lion), others are independent of my will—I do not choose, when I open my eyes, what I see: There is therefore some other Will or Spirit that produces them ⁷. The fixed rules governing this production are the laws of nature, which we learn through experience.

So we behold this consistent uniform working, which so evidently displays the Goodness and Wisdom of that governing Spirit whose Will constitutes the Laws of Nature ⁸. It is the Author of Nature ⁹—as active intelligence—who is the true cause of phenomena. Thus, fire is not the cause of heat (nothing could be more absurd, in Berkeley's view); it is this intelligence that causes heat.

Within this conception, we can distinguish between ideas and "real" things: if real things are Ideas imprinted on the Senses by the Author of Nature, our imagined ideas are excited in the Imagination ¹⁰.

The former have more reality than the latter, being stronger, more orderly, and more coherent, but this is no Argument that they exist without the Mind ¹¹.

Berkeley anticipates an objection: his doctrine seems to render the world chimerical, implying that we perceive only imaginary things existing solely in our minds.

Berkeley replies that this is not so. In his theory, the Distinction between Realities and Chimeras retains its full force ¹². This distinction is precisely what he has just drawn: realities are more coherent, more ordered ideas that obey the laws of nature. In fact, the things I see with mine Eyes and touch with my Hands do exist, really exist [...]. The only thing whose Existence we deny, is that which Philosophers call Matter or corporeal Substance. ¹³.

Berkeley is thus not advocating a new form of scepticism that denies the reality of the world: The Sun that I see by Day is the real Sun ¹⁴. More generally, we are not for having any Man turn Sceptic, and disbelieve his Senses; on the contrary we give them all the Stress and Assurance imaginable; nor are there any Principles more opposite to Scepticism, than those we have laid down ¹⁵, since what I see, hear and feel doth exist ¹⁶.

¹ §23
² ibid.
³ §25
⁴ §26
⁵ ibid.
⁶ §27
⁷ §29
⁸ §32
⁹ §33
¹⁰ ibid.
¹¹ ibid.
¹² §34
¹³ §35
¹⁴ §36
¹⁵ §40
¹⁶ ibid.