Enlightenment began with an unparalleled confidence in human reason. The new science’s success in making clear the natural world through Locke, Berkeley, and Hume affected the efforts of philosophy in two ways. The first is by locating the basis of human knowledge in the human mind and its encounter with the physical world. Second is by directing philosophy’s attention to an analysis of the mind that was capable of such cognitive success.
John Locke set the tone for enlightenment by affirming the foundational principle of empiricism: There is nothing in the intellect that was not previously in the senses. Locke could not accept the Cartesian rationalist belief in innate ideas. According to Locke, all knowledge of the world must ultimately rest on man’s sensory experience. The mind arrives at sound conclusions through reflection after sensation. In other words the mind combines and compounds sensory impressions or “ideas” into more complex concepts building it’s conceptual understanding.
There was skepticism in the empiricist position mainly from the rationalist orientation. Locke recognized there was no guarantee that all human ideas of things genuinely resembled the external objects they were suppose to represent. He also realized he could not reduce all complex ideas, such as substance, to sensations. He did know there were three factors in the process of human knowledge: the mind, the physical object, and the perception or idea in the mind that represents that object.
Locke, however, attempted a partial solution to such problems. He did this by making the distinction between primary and secondary qualities. Primary qualities produce ideas that are simply consequences of the subject’s perceptual apparatus. With focusing on the Primary qualities it is thought that science can gain reliable knowledge of the material world. Locke fought off skepticism with the argument that in the end both types of qualities must be regarded as experiences of the mind.
Lockes Doctrine of Representation was therefore undefendable. According to Berkley’s analysis all human experience is phenomenal, limited to appearances in the mind. One’s perception of nature is one’s mental experience of nature, making all sense data “objects for the mind” and not representations of material substances. In effect while Locke had reduced all mental contents to an ultimate basis in sensation, Berkeley now further reduced all sense data to mental contents.
The distinction, by Locke, between qualities that belong to the mind and qualities that belong to matter could not be sustained. Berkeley sought to overcome the contemporary tendency toward “atheistic Materialism” which he felt arose without just cause with modern science. The empiricist correctly aims that all knowledge rests on experience. In the end, however, Berkeley pointed out that experience is nothing more than experience. All representations, mentally, of supposed substances, materially, are as a final result ideas in the mind presuming that the existence of a material world external to the mind as an unwarranted assumption. The idea is that “to be” does not mean “to be a material substance;” rather “to be” means “to be perceived by a mind.”
Through this Berkeley held that the individual mind does not subjectively determine its experience of the world. The reason that different individuals continually percieve a similar world and that a reliable order inheres in that world is that the world and its order depend on a mind that transcends individual minds and is universal (God’s mind). The universal mind produces sensory ideas in individual minds according to certain regularities such as the “laws of nature.” Berkeley strived to preserve the empiricist orientation and solve Lockes representation problems, while also preserving a spiritual foundation for human experience.
Just as Berkeley followed Locke, so did David Hume of Berkeley. Hume drove the empiricist epistemological critique to its final extreme by using Berkeley’s insight only turning it in a direction more characteristic of the modern mind. Being an empiricist who grounded all human knowledge in sense experience, Hume agreed with Lockes general idea, and too with Berkeley’s criticism of Lockes theory of representation, but disagreed with Berkeley’s idealist solution. Behind Hume’s analysis is this thought: Human experience was indeed of the phenomenal only, of sense impressions, but there was no way to ascertain what was beyond the sense impressions, spiritual or otherwise.
To start his analysis, Hume distinguished between sensory impressions and ideas. Sensory impressions being the basis of any knowledge coming with a force of liveliness and ideas being faint copies of those impressions. The question is then asked, What causes the sensory impression? Hume answered None. If the mind analyzes it’s experience without preconception, it must recognize that in fact all its supposed knowledge is based on a continuous chaotic volley of discrete sensations, and that on these sensations the mind imposes an order of its own. The mind can’t really know what causes the sensations because it never experiences “cause” as a sensation. What the mind does experience is simple impressions, through an association of ideas the mind assumes a causal relation that really has no basis in a sensory impression. Man can not assume to know what exists beyond the impressions in his mind that his knowledge is based on.
Part of Hume’s intention was to disprove the metaphysical claims of philosophical rationalism and its deductive logic. According to Hume, two kinds of propositions are possible. One view is based purely on sensation while the other purely on intellect. Propositions based on sensation are always with matters of concrete fact that can also be contingent. “It is raining outside” is a proposition based on sensation because it is concrete in that it is in fact raining out and contingent in the fact that it could be different outside like sunny, but it is not. In contrast to that a proposition based on intellect concerns relations between concepts that are always necessary like “all squares have four equal sides.” But the truths of pure reason are necessary only because they exist in a self contained system with no mandatory reference to the external world. Only logical definition makes them true by making explicit what is implicit in their own terms, and these can claim no necessary relation to the nature of things. So, the only truths of which pure reason is capable are redundant. Truth cannot be asserted by reason alone for the ultimate nature of things. For Hume, metaphysics was just an exalted form of mythology, of no relevance to the real world.
A more disturbing consequence of Hume’s analysis was its undermining of empirical science itself. The mind’s logical progress from many particulars to a universal certainty could never be absolutely legitimated. Just because event B has always been seen to follow event A in the past, that does not mean it will always do so in the future. Any acceptance of that “law” is only an ingrained psychological persuasion, not a logical certainty. The causal necessity that is apparent in phenomena is the necessity only of conviction subjectively, of human imagination controlled by its regular association of ideas. It has no objective basis. The regularity of events can be perceived, however, there necessity can not. The result is nothing more than a subjective feeling brought on by the experience of apparent regularity. Science is possible, but of the phenomenal only, determined by human psychology.
With Hume, the festering empiricist stress on sense perception was brought to its ultimate extreme, in which only the volley and chaos of those perceptions exist, and any order imposed on those perceptions was arbitrary, human, and without objective foundation. For Hume all human knowledge had to be regarded as opinion and he held that ideas were faint copies of sensory impressions instead of vice – versa. Not only was the human mind less than perfect, it could never claim access to the world’s order, which could not be said to exist apart from the mind. Locke had retained a certain faith in the capacity of the human mind to grasp, however imperfectly, the general outlines of an external world by means of combining operations. With Berkeley, there had been no necessary material basis for experience, though the mind had retained a certain independent spiritual power derived from God’s mind, and the world experienced by the mind derived its order from the same source.