David Hume (1711-1776) was a Scottish philosopher and historian who promoted empiricism and naturalism over spirituality. However, the Stanford Encylopedia of Philosophy states the following regarding Hume: "Although many of Hume's own contemporaries were happy to label Hume an 'atheist', our own contemporaries are more divided on this issue." He was skeptical towards religion, and his major philosophical works include A Treatise on Human Nature, An Enquiry Concerning the Principles of Morals, and Dialogues Concerning Natural Religion. He also wrote an attack against the occurrence of miracles, though it is now believed to be somewhat tautological, and the wide-ranging History of England, which covered the history of Great Britain from the time of Julius Caesar to 1688. Many of his quotes have survived to this day, including "You can tell what is inside a person's soul by what comes out if it."
Hume has been criticized by many theists for his atheistic approach, and Charles Darwin declared Hume to have been his central influence, and "Darwin's bulldog" Thomas Henry Huxley admired him so much that he wrote a book about him called Hume. Furthermore, Hume was denied several academic positions - such as the coveted chair of moral philosophy at the University of Edinburgh - possibly because he was so commonly assumed to be irreligious and much of morality was considered to come from God. One admirer of Hume was Adam Smith who, upon Hume's death in 1776, wrote the following to his friend William Strahan:
"Upon the whole, I have always considered him [Hume] both in his lifetime and since his death, as approaching as nearly to the idea of a perfectly wise and virtuous man, as perhaps the nature of human frailty will permit."
Hume is well-known for his commitment to empiricism. He believed that all meaningful ideas (beliefs, abstractions and general cognitive events) were traceable to some sense impression. For instance, according to Hume, my belief that there is a Ming vase on my dresser is meaningful if and only if that belief depends upon my having sensed such an object at that location. If I cannot find some sense perception of that object, according to Hume, I have no justification for my belief and should abandon it.
Writing about metaphysics, Hume charges most philosophers with logical incontinence. He attacks the metaphysical principle of causation (the view that causal laws are necessary relations between events), arguing that there could be no sense impressions that would justify such a belief. All that we may infer from our senses are that certain events are "constantly conjoined." But the constant conjunction of X and Y does not logically imply a necessary connection between X and Y. For instance, we have always seen the sun rise in the morning, but we cannot show that it must do so tomorrow. For all we know, it shall not.
It is important to note that Hume is not a radical skeptic. He is not worried that the sun shall not rise tomorrow; rather, he is simply remarking on our inability to have absolute certainty about the world based solely on our empirical observations of it.
We tend to believe that some objects endure over time and exist regardless of whether or not we currently perceive them. But, as Hume notes, if we are empiricists we cannot trace that belief to any impressions: it is a logical contradiction to say that we might have sense impressions of objects we do not perceive (a "perception" is the same thing as a sense impression). Thus, our belief in the endurance of objects seems to be a fiction. Again, however, it is important to note that Hume does not claim that objects have no existence external to the mind, or that objects do not in fact endure. Hume is just reminding us that we cannot claim to be certain of it.
Influence on Kant
Hume's work had an immediate influence on the German philosopher Immanuel Kant, whose Critique of Pure Reason was in some respects a response to Hume's empiricism. Drawing on Hume's observations about the limits of the human mind (specifically, the limitations on how it perceives reality), Kant claimed that Hume's empiricism leads to idealism - the view that some or all objects only have existence in the mind. Kant reverses this skeptical claim, arguing that the categories of spatial position, time and objectivity are imposed by the mind on our perceptions in order that we may make sense of them. He called this view transcendental idealism because, from the standpoint of the observer, this type of idealism necessitates the truth of our empirical claims relating to the categories he included in the reversal. In other words, because the category of "space" is a cognitive construct, we may make claims about the spatial relations of objects with certainty because the relations we are describing are nothing more or less than our ideas of them.
Hume, David. A Treatise on Human Nature Smith, Adam. "Letter to William Strahan" November 9, 1776