Empiricism is a posteriori, any knowledge is dependent upon the senses, observation, and experience, and apart from reason, but not necessarily being certain or justified knowledge other than any knowledge gained from induction. If there is any certainty that may have been gained through the history of philosophy is that not everyone knows everything, some may know something, but not many know anything.
Understanding David Hume’s skepticism may be easier to grasp by understanding his intent. A misconception may be that Hume was attempting to deny any kind of certainty any previous philosopher may have developed. The intent behind Hume’s skeptical doubts was not to discourage but rather to stimulate himself and others to develop a more satisfying system of achieving certainty, (Morris, Brown). Although Hume was an empiricist, he did not deny that certainty could not be gained by reason. Bertrand Russell claims that Hume believed algebra and arithmetic are the only sciences which may infer certainty through reason, (663).
There are four important subjects to understand before arriving at Hume’s skepticism in relation to causality; perceptions, impressions, ideas, and his established maxim. There is nothing present in the mind of anyone other than their own perceptions. Perceptions are significant to everyone’s experiences of their everyday life. Perceptions are better understood in two ways, impressions, and ideas. The impressions in the mind are when one, “hears, or sees, or feels, or loves, or hates, or desires, or wills . . . ideas are what is in the mind when one reflects on a passion or an object which is not present,” (Strauss). Impressions and ideas are both in the mind; however, impressions are in the mind when one experiences a present object through their sensory perceptions. Ideas are also present in the mind but are direct reflections people have from an object which is not in front of them. Ideas can also be reflections one has from their impressions. For example, if someone picks up an object in front of them like a gun; they can see, feel, hear, or it may ignite a sense of passion in the individual perceiving it. Whatever the individual is thinking in the mind are impressions from the object in front of them. An example of an idea someone has is when someone for any reason thinks of an object, not in front of them; for example, the gun may cause the individual to reflect on a good or bad memory, it may motivate the individual to think of laws, safety, training, hunting, etc. What ideas and impressions have in common is that they are both perceptions in the mind and based on experience alone, not reason or rationality. Having understood perceptions, impressions, and ideas; it may be inferred that nothing can be conceived in the mind that has not been experienced.
The final subject one should understand is the “established maxim” which may be thought of as an imperative. The “established maxim” is anything that is conceivable is possible. Any assertion which cannot be imagined as contrary to a fact in the mind is knowledge, and any assertion which can be imagined as contrary to a fact in the mind is not knowledge, (Strauss). For example, that a square has four corners is a logical fact. To attempt to conceive in the mind the contrary of a square with four corners; for example, a square circle is logically impossible and therefore not knowledge. If one can conceive in the mind something other than the logically impossible such as; gravity is an active function at any given moment is a fact, but it is possible to imagine the contrary to the fact, that the function of gravity tomorrow morning may cease as a function, and since it is conceivable in the mind it is a possible fact that it may occur.
These I believe are the keys to understanding Hume’s skepticism. Perceptions, impressions, ideas, are all based on empiricism to Hume, and anything empirical or anything based on sensory perception or experience may be doubted. All perceived knowledge may be justifiably doubted because even any reason and rationality considered as knowledge, yet may be conceived or imagined as contrary to the supposed fact, is now a probable error. Apply these principles to all-cause and effect situations and you have justifiable skepticism.
Morris E.William., Brown R. Charlotte. David Hume. Stanford Encyclopedia of Philosophy. 21, May 2014. https://plato.stanford.edu/entries/hume/#Cau. Web. 06, Dec. 2018.
Russell, Bertrand. The History of Western Philosophy. New York. Simon & Schuster Inc. 1972. Print.
Strauss, Leo., Cropsey, Joseph. History of Political Philosophy. Chicago. The University of Chicago Press. 1987. Print.