Locke argues that there is a crucial difference between the primary and secondary qualities, the kinds of simple ideas that people receive from feeling. Some of the ideas do not resemble their causes in the world, while others do. The ideas that do resemble their primary qualities: motion, number, shape, etc. The ideas of the secondary qualities are the ones of sounds, color, smell, etc. The simplest way to understand the differentiation of the two qualities lies in the explanation. For instance, if there is a sensation from an object, the explanation for this sensation is the shape of the object. If a person has a sensation of color, on the other hand, the sensation is connected with the specific arrangements of the parts of the matter that cannot be sensed.
The most famous indirect argument for idealism from George Berkeley is closely connected with the opposition of the theory of primary and secondary qualities. This theory, as proposed by John Locke, was a mainstream theory Berkeley wanted to demolish. Berkeley argued that the same relativity of perception arguments that demonstrate that secondary qualities are not in bodies but rather in the perceiver, apply equally to primary qualities such as size and motion. His core argument was the opinion that people can perceive common objects (cars, trees, rivers), as well as they, can perceive ideas, therefore, common objects are equal to ideas. As presented in Principles, Berkeley campaigned against matter, arguing that everything a person can ever perceive directly or indirectly is idea.
Locke never doubted the existence of matter, while Berkeley posed a concrete question: “If substance is an assumption, then could that assumption be wrong?” I suppose that the material world does not exist, then how could people account for the supposed objects that cause them to perceive. Therefore, according to Berkeley, to be meant to be perceived, and not to be perceived means not to exist. In his opinion, there are no “real” cars, trees, or rivers; these objects exist in perception, as they only exist while being perceived. Furthermore, Berkeley concludes that there is no actual distinction between the primary and secondary qualities, as all qualities are in the mind. When it comes to whether the material world, in Berkeley’s opinion it’s completely false as no evidence demonstrates its validity.
In my opinion, Berkeley does not prove that the material world is fiction, although when it comes to his sceptical arguments, they hold the most water. Another weak argument proposed by Berkeley is his idea that objects that people perceive only exist while being perceived, however, he forgets the common-sense fact that objects exist while not being perceived by one person, this problem had received very little attention. In the light of attacks on Locke’s realism, the strongest case made by Berkeley is his reduction of the physical aspects of the world to the sets of ideas that solve some logical problems of knowledge in the world.
Any idea proposed by philosophers can argue against it, so there is no definite answer on whether the material world exists or not. No matter what Locke or Berkeley said in their works, it’s all in individual perception.