David Hume
An Enquiry concerning Human Understanding
Hume’s Skepticism
Reject everything that is not directly known or knowable by experience, in particular,
no spiritual substances or occult (hidden) powers.
Goal: to make philosophy scientific, like Newton’s physics.
Hume’s hostility to religion, which he regarded as superstitious.
Hostility to metaphysics—that is, to any idea of non-material beings. Philosophy must
not open the door for God, angels or an immortal, immaterial human soul.
The Origin of Ideas
“Everyone will readily allow that there is a considerable difference between the
perceptions of the mind when a man feels the pain of excessive heat or the
pleasure of moderate warmth, and when he afterwards recalls to his memory this
sensation or anticipates it by his imagination.”

Thoughts or ideas: less vivid and forcible.
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Impressions: vivid & lively, presented by the senses.
Ideas (or thoughts) come from impressions.
Combining Ideas
“Nothing, at first view may seem more unbounded than the thought of man
which not only escapes all human power and authority, but is not even
restrained within the limits of nature and reality.”
“Or, to express myself in philosophical language, all our ideas or more feeble
perceptions are copies of our impressions, or more lively ones.”
First argument for this principle
Simple ideas come directly from sense impressions. All our thoughts or ideas resolve
into simple ideas. Even the biggest idea—God—comes from “reflecting on the operations of our own
mind and augmenting, without limit, those qualities of goodness and wisdom”
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Second Argument
If a man is born without a particular organ of sensation, he is not susceptible to the
corresponding ideas. “A blind man can form no notion of colors; a deaf man of sounds.” Restore the
sense, and you open a new inlet for fresh ideas.
Consequence of this principle
“When we entertain, therefore, any suspicion that a philosophical term is
employed without any meaning or idea…, we need but enquire, from what
impression is that supposed idea derived?”
“By bringing ideas into so clear a light we may reasonably hope to remove all
dispute which may arise concerning their nature and reality.”
What can be known?
“All objects of human reason or enquiry may naturally be divided into two
kinds, to wit, relations of ideas and matters of fact.”
Relations of ideas: mathematics
 The Pythagorean theorem

3 x 5=½ x 30
These truths don’t depend on any existing thing. They are always and everywhere true.
Matters of fact
These are not determined by pure reason. The contrary of every matter of fact is still
possible. That the sun will not rise tomorrow is not self-contradictory. We can imagine it. So what
makes anything true, beyond the testimony of our senses?
What is Hume worried about?
Descartes said that he proved he had an immaterial thinking self. William Paley said
that the order of the universe proved there is a God to put it in order. Many believed that the
laws of physics were absolute. But absolute laws require an absolute Lawgiver—a God.
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Hume wants to show that we can’t really know anything except what we can
experience with our senses.
The Challenge: Cause & Effect
The watch proves people were on the island. So, does the harmony of the universe
prove that God exists? I know my friend is in France, because I saw a letter from him. So,
from the existence of ancient Scriptures can I know that God exists?
Are cause-effect arguments a priori and necessary?
Causal arguments not a priori
“I shall venture to affirm, as a general proposition which admits of no
exception, that the knowledge of this relation is not, in any instance, attained by
reasonings a priori, but arises entirely from experience, when we find that any
particular objects are constantly conjoined with each other.”
Causes & Effects
“Causes and effects are discoverable not by reason but by experience.”
Who would guess that two smooth pieces of marble would stick tightly together? Or
that a lodestone (natural magnet) would attract iron? “Who can assert that he can give the
ultimate reason why milk or bread is proper nourishment for a man, not for a lion or a tiger?”
The real basis for causal argument
Not reason, but custom: The effect can never be found in the cause, not in the billiard
ball nor in the falling stone. There is no a priori law of cause & effect.
“Every effect is a distinct event from its cause.”
The basis for scientific reasoning
We know that bread nourishes, but there is no known connection between its sensible
properties and its nutritional powers. Only experience—not reason—tells us that bread is
nutritious. We predict that a loaf of bread will nourish, not on the basis of reason, but because
that has been our experience.
The Logical Problem
“These two propositions are far from being the same,
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1. I have found that such an object has always been attended with such an effect,
and
2. I foresee that other objects which are in appearance similar will be attended
with similar effects.”
We recognize this inference as valid, but why?
No Logical Connection
The mind naturally and normally moves from past experiences to future expectations.
We all do this…all the time. But—Hume’s point—the connection of experience to
expectation is not one of reason. “But if you insist that the inference is made by a chain of
reasoning, I desire you to produce that reasoning.”
In other words…
It’s perfectly reasonable to expect the sun to rise tomorrow or to see rain when dark
clouds roll in. BUT there is no eternal law of reason that requires these things to happen.
There is no causal law…only patterns that we get used to.
No Hidden Powers
Someone completely new to the world would not know what to expect from it.
Nothing in his growing experience reveals the secret powers by which one thing causes
another.
“He would not, at first, by any reasoning, be able to reach the idea of cause and
effect; since the particular powers by which all natural operations are performed
never appear to the senses.”
Custom and Habit
“Custom, then is the great guide of human life. It is that principle alone which
renders our experience useful to us …”
“If I ask why you believe any particular matter of fact which you relate, you
must tell me some reason; and this reason will be another fact, connected with
it. But as you cannot proceed in the manner ad infinitum, you must at last
terminate in some fact which is present to your memory or senses; or must allow
that your belief is entirely without foundation.”
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In Conclusion
“All belief of matter of fact or real existence is derived merely from some object
present to the memory or senses, and a customary conjunction between that and
some other object.”
“All these operations [of the mind] are a species of natural instincts, which no
reasoning of process of thought can understanding is able either to produce or
to prevent.”
Hume’s importance

A ‘ruthless’ empiricist: if an idea cannot be traced back to sense experience, it
is useless and false.

Foundation for the belief that only the physical sciences can give us truth.

So religion, ethics, and culture have nothing to do with objective reality.
Strongly anti-metaphysical, he had a profound effect on philosophy in the first half of
the 20th Century, especially in England and the US.
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