In the "Principles of Human Knowledge," George Berkeley argues that material objects are nothing more than images within the mind. This idealism suggests that no physical non-thinking entities are real. The only reality of the objects is the image within the mind of the perceiver of the objects. For instance, objects are perceived by the senses. We then perceive or interpret our own ideas or sensations. Thus, the ideas have more reality to us than does the objects that we supposedly perceive.
Next, Berkeley defines the term "existence." For a body to exist, one must perceive it or have the potential to perceive it. Yet, everything that we perceive or have the potential to perceive is ideas. Thus, to say that a body exists is to say that you merely had an idea of that body. An objection to this argument is that one cannot have an idea of something without perceiving the real object. To retort this statement, Berkeley suggests that we cannot distinguish the sensible objects form the perception of these objects.
In other words, it is impossible to separate, in thought, the so called real bodies that we see and feel from the mental perception of these bodies. Berkeley asserts that to separate these images would be analogous to dividing a human body or separating the image of a rose from the smell of a rose. According to Berkeley, separating these images would be an example of abstraction. In his effort to prove that material objects only exist within the mind of the perceiver, Berkeley asserts that one must actually perceive an object to have an idea about it. An idea exists only if it is perceived. Physical objects are collections of ideas.
Physical objects can therefore only exist when perceived. For example, consider a person to be thinking of a certain tree in his favorite pasture. If he does not actually look at this tree or directly sense it, then his idea of the tree is based only on a previous idea or image o the tree. The recollection of former ideas does not constitute the existence of an object. Although only ideas themselves, the senses play a key role in the arguments for Berkeley's idealism, such as in this quick proof for the nonexistence of the qualities of bodies. Berkeley defines sensible qualities of bodies to be such things as color, taste, figure, and motion.
These qualities are perceived by the senses. The senses perceive ideas. Therefore, the qualities of bodies are ideas. Berkeley furthers his defense for this view by arguing that numbers and certain senses are ideas within the mind.
He states that numbers only exist within the mind because they depend entirely on the understanding of man. Similarly, heat, cold, colors and other sensible qualities are nothing more than affections of the mind. These sensible qualities are not a part of matter and could only exist if someone is there to experience the sensation. One of Berkeley's most crucial arguments for the nonexistence of active bodies lies in his explanation of why actions can exist only within the mind.
First, it is given that ideas exist only within the mind. As proven earlier, ideas are all that is perceived. An idea is defined as passive in nature. This means that an idea cannot directly do anything and cannot be the direct cause of anything. Thus, an idea cannot be the resemblance of an active being because an idea exists only as an idea. Therefore, we can conclude that extension, figure, and motion cannot cause sensation because these descriptions are merely ideas within the mind.
Berkeley continues his attack on the Matter that makes up objects. Matter is defined as "inert, senseless substance in which extension, figure, and motion do actually subsist" (258). Since extension, figure, and motion are ideas within the mind, matter must also be an idea because it is defined by extension, figure, and motion. Likewise, one cannot perceive of matter without using the senses, which exist only in the mind. Berkeley asks us to suppose for a moment that matter exists in reality but that the sensible or "secondary qualities" (258) exist only within the mind. The physical body and the secondary qualities would be utterly inseparable from each other because both were needed to comprise the object.
Berkeley then proposes that the sensible ideas cannot exist only within the mind while the objects exist only in reality if the two are entirely inseparable. Thus, the objects must exist within the mind as well. At the conclusion of his arguments, Berkeley depicts the entities that exist within the mind of man. He explains that a spirit causes ideas within the mind.
The "understanding" perceives the ideas while the "will" operates or produces ideas. Also, Berkeley describes "laws of nature" which are the methods and patterns that are perceived by the idea of senses. These laws may be the only real ideas because they are imprinted in us by the Author of Nature and teach us to associate certain ideas with others to learn how to live. 318.