The thinker John Locke tried to justify the origin of knowledge from the senses and developed the sensationalist theory of knowledge. All human ideas and concepts arise in the process of the impact of things that exist outside of us and independent of us, our senses. The world of objects and things around us determines the content of human sensations. In this regard, Locke notes, there is nothing in the mind that was not previously in our senses. He explores the forms of cognition and considers the question of the sources of the formation of ideas and concepts. Promoting experience as the only source of all thoughts, the philosopher divides it into external and internal.
Locke believes that external experience and perception give ideas of such qualities that belong to the objects and are in them as they are in our sensations. Locke defines these qualities as objective, primary qualities; they are inseparable from objects and permanently-preserved in them. These are extension, motion, density, figure, and number. Secondary qualities are subjective; they do not belong to the things of the material world themselves. In the latter, there is only the ability to cause these sensations. The secondary group includes sound, taste, color, and smell, which do not belong to the objects of the external world. From this point of view, Locke believes that if there are no objects capable of hearing the sound of a fallen tree, the latter does not exist. At the same time, the tree exists even if no one can feel it because primary qualities are present: density, length, and movement of branches from the tree; all these properties are primary and, therefore, constant.
George Berkeley exposes Locke’s ideas by pointing out that there are neither primary nor secondary qualities, for they are perceptible. Therefore, the things represented through these properties are conditioned by the perceiving subject. Berkeley emphasizes that there is no need to recognize two kinds of qualities, for even assuming their separate existence from each other, scientists would have to admit that they have a different nature. However, the things which coincide in nature are the same; hence, it is fair to assume that the bodies are complexes of sensations. The philosopher refers to the beginnings of geometrical optics, he criticizes it, noting that all people perceive everything differently, so relying on geometrical optics is not entirely advantageous and correct. Berkeley leads to the idea that the primary qualities depend on the subject, on its state. Therefore, these primary qualities themselves can in no way substantiate things in their material independence from human perception; correspondingly, no matter what substance exists and cannot exist, everything can exist only in sensation.
Berkeley’s empirical conclusion that there is no permanently existing reality, and his paradoxical resolution of this question by reference to an omnipresent God, is close to the concept of sophistry. Nevertheless, in his approach to the relation to the tree, his point of view is more accurate. Berkeley’s thought that the perception of any properties of an object depends on the subject of observation is close to what was proved centuries later by physicists. From the point of view of modern science, Locke’s theory is not logical because the speed attributed by the philosopher to the primary quality is perceived differently by different people, depending on the rate at which the observer moves. Consequently, this indicator cannot be constant like the other primary qualities.