This book largely seeks to refute the claims made by Berkeley's contemporary John Locke about the nature of human perception.

At the end of his Introduction, he advised the reader to let his words engender clear, particular ideas instead of trying to associate them with non–existent abstractions.

[8] According to Locke, a thing's primary qualities, such as its extension, shape, motion, solidity, and number, exist unperceived, apart from any perceiver's mind, in an inert, senseless substance called matter.

The greatness and smallness of figure, the swiftness and slowness of motion, exist in the mind and depend on point of view or position.

Berkeley declared that it is "…impossible that any colour or extension at all, or other sensible quality whatsoever, should exist in an unthinking subject without the mind, or in truth, that there should be any such thing as an outward object.

"[16] Any quality that depends on sensation for its existence requires that a sense organ and a mind is conscious of it.

By "unthinking subject," he means "mindless matter" or "substance, substratum, or support that is not a thinking mind."

Through reflection or introspection it is possible to attempt to know if a sound, shape, movement, or color can exist unperceived by a mind.

Berkeley declared that he will surrender and admit the unperceived existence of material objects, even though this doctrine is unprovable and useless, if "…you can conceive it possible for one extended moveable substance or, in general, for any one idea, or anything like an idea, to exist otherwise than in a mind perceiving it….

"To say, therefore, that these [sensations] are the effects of powers resulting from the configuration, number, motion, and size of corpuscles must certainly be false.

Berkeley claimed that a person's active mind can imaginatively generate ideas at will.

Objection: [A]ll that is real and substantial in nature is banished out of the world, and instead thereof a chimerical scheme of ideas takes place.

Real things are more strongly affecting, steady, orderly, distinct, and independent of the perceiver than imaginary chimeras, but both are ideas.

Answer: Berkeley requests that the reader…consider whether he means anything by the actual existence of an idea distinct from its being perceived.

"[16] "[I]t is the mind that frames all that variety of bodies which compose the visible world, any one whereof does not exist longer than it is perceived.

[45] Answer: People notice that some ideas appear in their minds independently of their wishes or desires.

The most important reason why philosophers do not consider God ("Supreme Spirit) as the only possible cause of our perceptions, is "because His operations are regular and uniform".

Order and concatenation of things are "an argument of the greatest wisdom, power and goodness in their Creator".

It would be less effort for objects to appear as ideas with simple exterior surfaces, without so many internal connections.

Objection: Matter may possibly exist as an inert, thoughtless substance, or occasion, of ideas.

[49] Answer: If matter is an unknown support for qualities such as figure, motion and color, then it doesn't concern us.

Objection: Holy Scripture speaks of real things such as mountains, cities, and human bodies.

[64] Every particular finite line, surface, or solid which may possibly be the object of our thought is an idea existing only in the mind, and consequently each part of it must be perceived.

In order to use mathematics, it is not necessary to assume that there are infinite parts of finite lines or any quantities smaller than the smallest that can be sensed.

[70] When I perceive the order and harmony of nature, I know that God, as infinitely wise spirit or mind, is the cause.

[72] Following a line of thought which can be traced back to Augustine's Theodicy, Berkeley argues that imperfections in nature, such as floods, blights, monstrous births, etc., are absolutely necessary.

They are the result of the working of the system of simple, general, consistent rules that God has established in nature in order that living things can survive.

When seen from a higher, broader perspective, particular evils are known to be good when they are comprehended as parts of a beautiful, orderly whole system.

If we are clearly convinced of God's existence, then we will fill our hearts with awful circumspection and holy fear.

If we think that the eyes of the Lord are everywhere, beholding the evil and the good, knowing our innermost thoughts, then we will realize our total dependence on Him.