An Enquiry Concerning Human Understanding

David Hume was born in the eighteenth century, slap-bang in the middle of the Enlightenment. With the scientific method ascending to prominence in Europe, it was a time of unparalleled optimism in the power of reason to discover truth.

Over the course of this fruitful period, many great thinkers like Descartes, Locke, and Berkeley came up with novel ways to try and ground their philosophies in reason. But, despite their different approaches, all these philosophers made the same fundamental error – they all attempted to shoehorn their theological commitments into their rational philosophies.

In many respects, Hume’s empirical philosophy responds to these developments. On the one hand, Hume continued the rationalist thread of the Enlightenment by imploring us to justify our beliefs with reason. On the other hand, by limiting the scope of reason to the realm of human experience, Hume decoupled reason from the realm of speculation that requires theological beliefs, and thereby set the stage for a purely secular philosophy.

The key message here is: All knowledge derives from experience.

Hume’s entire empirical philosophy is built on one foundational distinction: the distinction between impressions and ideas.

Impressions are direct sensory experiences and emotions. For example, when you see the color red or when you feel angry, you’re experiencing direct impressions.

Ideas, on the other hand, come from the imagination or from memory. You have an idea when, for example, you close your eyes and conjure the color red or the feeling of anger. As they’re abstracted from direct experiences, we experience ideas as diluted and vaguer than the original impressions.

Now, since ideas are copies of impressions, it follows that we can’t have an idea until we’ve experienced the impression. You couldn’t, for example, really know what love is until you’ve actually been in love.

Of course, you might object that you have all sorts of ideas that you’ve never directly experienced. With help from the imagination, you can summon all manner of fictional worlds full of monsters and strange landscapes that you’ve never encountered in reality.

This is true. But, as Hume points out, the ideas of the imagination are all built out of simpler impressions that we’ve directly experienced. For example, we can imagine a mountain made of gold by mixing the simpler ideas of gold and mountain in our heads to form a new idea.

Since all ideas are derived from simple impressions, this provides us with a simple method for determining whether or not an abstract idea, such as God, has any meaning. All we need to do is point to the impression that produced it. If no impression can be found, then we have good reason to reject the idea as empty and groundless.