Locke is the founder of Modern Empiricism. He began his philosophy by attacking the Rationalist belief in "Innate Ideas" (ideas we are born with).
Locke argued that if ideas were innate (like the idea of God or the laws of logic), then children and "idiots" would know them, which they do not. He proposed that at birth, the mind is a Tabula Rasa (a blank slate).
"Let us then suppose the mind to be, as we say, white paper, void of all characters, without any ideas."
All knowledge comes from two sources: Sensation (external experience) and Reflection (the mind's internal operation on those sensations).
Locke distinguished between two types of qualities in objects to explain how we perceive the world:
Berkeley took Locke's Empiricism to its logical—and radical—conclusion. He argued that if secondary qualities exist only in the mind, then primary qualities do too.
Berkeley's famous dictum, "To be is to be perceived," means that "matter" does not exist. There are only Minds and Ideas. This is known as Subjective Idealism.
The Role of God: If objects only exist when perceived, why does a tree stay in the quad when no human is there? Berkeley answered: Because God is the permanent perceiver who holds all ideas in existence.
Hume is the most consistent and radical of the empiricists. He divided the contents of the mind into two categories:
Hume applied his "Empiricist Razor": if an idea cannot be traced back to an impression, it is meaningless. This led him to doubt:
Q: How does Berkeley differ from Locke?
A: Locke believes in a material world (Substance) that causes our ideas. Berkeley denies the existence of matter entirely, claiming only spirits and ideas exist.
Q: Why is Hume a 'Skeptic'?
A: Because he concludes that reason cannot prove the existence of the external world, the self, or the necessity of cause and effect.
In your answers, use the term "The Empiricist Bridge": Locke builds the bridge to the world, Berkeley burns the world-side of the bridge, and Hume burns the bridge entirely!