1. Skepticism David Hume’s Enquiry Concerning Human Understanding

2. Some terminology Necessary versus contingent truths
A necessary truth is something, the opposite of which is inconceivable without contradiction. For example that a bachelor is an unmarried man is a necessary truth. We can’t think of a married bachelor without having a contradiction. Another way to put it is that it couldn’t possibly be false, like 2+2=4.
A contingent truth is something where the opposite is conceivable without contradiction. For example, that Trump is the president of the United States is contingent; he could have done something else with his life, or someone else could have won instead.

3. Ideas versus Impressions
Ideas, for Hume, are the less lively “perceptions of the mind”. For example, recalling the colour of an apple you saw earlier.
Impressions, for Hume are the more active and lively sensory experiences, or actual perceptions. For example, actually seeing the colour of an apple is an example of an impression.
These are obviously distinguishable to us—given their differences in “force and vivacity” (14).

4. All ideas are traceable back to impressions
According to Hume, all of our ideas—even very complex ones—are basically built up of impressions.
The mind does not itself create qualities that it had not first perceived as impressions.
With the exception of the “missing shade of blue” case, which we can discuss.

5. How we connect ideas
Resemblance (an idea of something will lead us to think of something it resembles).
Contiguity in time or place (we think of things that occur together)
Cause and Effect (thinking of a cause will lead us to consider its effect)

6. Relations of Ideas versus Matters of Fact
Relations of ideas are operations of the mind—or things that are intuitively or demonstrably certain. Examples include mathematical truths; we can demonstrate their truth using purely mental operations.
Matters of fact, on the other hand, concern things that exist outside of us in the universe. To ascertain their truth we actually have to look to the world.

7. Hume thinks that all of our reasoning concerning matters of fact is founded on the relation of cause and effect.
For example, finding a watch on a deserted island would be taken as evidence for the fact that someone had been there (19).
“Here it is constantly supposed that there is a connexion between the present fact and that which is inferred from it. Were there nothing to bind them together, the inference would be entirely precarious” (19).

8. But, according to Hume, it isn’t clear what binds these sorts of inferences! And what does appear to bind them is faulty!

9. Hume’s main point
Hume’s main line of argument is best summed up by this passage:
“We have said that all arguments concerning existence are founded on the relation of cause and effect; that our knowledge of that relation is derived entirely from experience; and that all our experimental conclusions proceed upon the supposition that the future will be conformable to the past. To endeavour, therefore, the proof of the last supposition by probable arguments, or arguments regarding existence, must be evidently going in a circle, and taking that for granted, which is the very point in question” (26).

10. Basically, we justify our beliefs in certain events happening the way that we are used to them happening because of our experiences. But we have no way of proving that just because something happened a certain way previously, it will happen again.
We are taking for granted that “the future will be conformable to the past” (26). However, we don’t actually have good philosophical reason to do so.

11. What justifies our inferences that we expect the same effects from the same sorts of causes?
Experience…but this seems circular! For example: “I know the sun will rise tomorrow because the sun has always risen in the past”.
Hume is pointing out that we have no outside reason to expect uniformity in our experiences and in nature. We just assume it based on what happened in the past.

12. It is conceivable that, for example, the sun not rise tomorrow—but we expect that it will, given that it has always risen in the past.
This doesn’t mean that we should seriously doubt that the sun will rise tomorrow, but Hume thinks that we tend to infer these types of things is philosophically interesting.

13. There is no necessary connection between effects and their causes.
We can’t look at what is commonly thought of as the cause of X, and know prior to experience that X will follow from it.
For example, how do we know that bread will provide us with nourishment? Because it has done so in the past. But there is nothing in the observable qualities of bread that can tell us that it will provide nourishment.

14. Causes and their effects appear to be arbitrarily connected by relations of the mind. So, is there really such a thing as causality???
We can’t discover effects in their causes. Is that really a problem?
Are we right to expect that the future will be conformable to the past?

15. Pollock
Pollock raises the issue of skepticism regarding our experiences of reality.
How do we know that our experiences are real and not simulated?
Is this a serious worry for people?

16. Pollock’s little scenario is an example of a skeptical hypothesis
Pollock’s little scenario is an example of a skeptical hypothesis. These are used sometimes in philosophy to show that we can’t actually be certain about anything, even those things we take to be mundane and uncontroversial.
What’s more, there doesn’t seem to be any way of refuting skeptical hypotheses. No matter what, you would still think and feel as though you were in control of your own reality!

17. The basic problem of skepticism
Skepticism generally concerns the problem of whether knowledge is possible at all.
Sometimes skepticism concerns whether knowledge in particular domains—such as whether the external world is real (like Pollock’s brain in a vat).

18. Responses to skepticism