1. John Locke ↑
English philosopher John Locke rejected French natural philosopher Rene Descartes' rationalism and, in 1690, he popularised Ancient Greek philosopher Aristotle's concept of the 'tabula rasa' once again. Like Aristotle, Locke argued that the mind does not have innate ideas and so sensory knowledge is the only knowledge we can have. This view is known as empiricism.
Locke claimed that if we had innate ideas - knowledge that does not come from experience - then all beings that possess a mind should be aware of them. Yet it's clearly true that people do not understand mathematics until they are taught, and some people are never able to learn.
Locke argued that if it's possible for a human mind to exist without being conscious of an idea, then it cannot be innate. Even if we could find some rational knowledge that everyone is aware of possessing, then Locke claimed this would still not show that we have come to know these ideas innately and not through shared experiences.
Locke argued that we have two types of experiences: sensations and reflections. We gain some knowledge from reflection, some from sensation, and some from both.
In An Essay Concerning Human Understanding, Locke described reflection as "that notice which the mind takes of its own operations, and the manner of them". Reflection allows us to be conscious of our mental processes, and so tells us about how our minds operate. Examples of reflection include "thinking, doubting, believing, reasoning, knowing", and "willing". These experiences all invoke qualia that do not correspond to external objects, and so Locke referred to reflection as an "internal sense"[1a].
Sensations arise from external stimulus and tell us about the external world. Locke described two types of sensations: those corresponding to primary, and secondary, qualities. Primary qualities are similar to the properties Descartes acquainted with rational introspection, such as size, shape, and quantity. Secondary qualities correspond to qualia, such as colour, sound, and emotion. Locke highlighted the problem of secondary qualities with his example of the inverted spectrum.
Like Descartes, Locke claimed that it's impossible to know if different people experience the same qualia:
"if the idea that a violet produced in one [person’s] mind by [their] eyes were what a marigold produced in another [person’s], and vice versa. This could never be known, because one [person’s] mind couldn't pass into another [person’s] body to perceive what appearances were produced"[1b].
This assumes that different people could experience different colours despite exhibiting the same behaviour and brain activity.
Violet and marigold qualia. Image credit: Helen Klus/CC-NC-SA.
Instead, Locke advocated causal realism, the view that we can at least derive the existence of external objects from the qualia they invoke in us. Specifically, Locke believed that an object's primary qualities are representative of its true nature, and that they are responsible for inducing the secondary qualities we experience in their presence. We cannot know if these objects really resemble the qualia they invoke.
Like English philosopher Thomas Hobbes, Locke did not think we are capable of understanding how external objects give rise to qualia. He stated that:
"[Experience] convinces us, that we have an intuitive knowledge of our own existence and an internal infallible perception that we are. In every act of sensation, reasoning, or thinking, we are conscious to ourselves of our own being, and in this matter we don't fall short of the highest degree of certainty"[1c].
But this tells us nothing of the substance the mind is made of and so Locke did not accept Descartes' dualism. He suggested it's equally possible that the mind and body could be made of the same substance, leading to the idea that physical matter could be capable of thought.
2. George Berkeley ↑
In 1710, twenty years after Locke first published his theory of knowledge, Irish empiricist George Berkeley criticised Locke's belief in causal realism, the view that we can determine the existence of the external world.
Berkeley argued that causal realism is inconsistent with empiricism, because it assumes there is a chain of causes, starting with the external object and ending with the secondary qualities we experience as qualia. Yet the brain only has access to the final stage, the qualia.
In A Treatise Concerning the Principles of Human Knowledge, Berkeley stated that:
"extension, figure, and motion are only ideas existing in the mind, and...an idea can be like nothing but another idea"[2a].
The qualia we perceive when we're awake are indistinguishable from the qualia we experience when we hallucinate or dream. This shows that qualia can be invoked without the existence of a causal link and, because there's no way to know when a causal link does or does not exist, Berkeley concluded that we have no reason to believe it exists at all.
Berkeley showed that if secondary qualities exist in the mind then primary qualities must also exist there, as we cannot imagine them devoid of qualia:
"extension, figure, and motion, abstracted from all other qualities, are inconceivable. Where therefore the other sensible qualities are, there must these be also, to wit, in the mind and nowhere else"[2b].
Berkeley claimed that we can't conceive of something existing in space without having a position and a velocity (which could be zero), yet it's evident that these are relative concepts and so originate in the mind. He went on to suggest that this must also be true of mass and numbers.
Berkeley stated that:
"number is so visibly relative, and dependent on men's understanding, that it is strange to think how any one should give it an absolute existence without the mind. We say one book, one page, one line, etc.; all these are equally units, though some contain several of the others. And in each instance, it is plain, the unit relates to some particular combination of ideas arbitrarily put together by the mind"[2c].
Berkeley rejected Descartes' dualism and Locke's agnosticism. Because everything that we experience originates in the mind, Berkeley claimed that the only theory available to empiricists is idealism, the view that physical objects do not exist.
Berkeley described the mind as "one simple, undivided, active being", and because nothing can exist without a mind to perceive it, he concluded that the external world must exist within the mind of God[2d].