Justified True Belief Michael Lacewing © Michael Lacewing

Three kinds of knowledge Acquaintance knowledge I know Oxford. Ability knowledge I know how to ride a bike. Propositional knowledge I know that elephants are heavier than mice.

Justified true belief Analyses knowledge in terms of necessary and sufficient conditions ‘I know that p’: The proposition ‘p’ is true; I believe that p; and My belief that p is justified.

Necessary and sufficient conditions Each condition is necessary for knowledge. The three conditions together are sufficient for knowledge.

The appeal of JTB I can’t know what is false. I can’t know a proposition that I don’t believe to be true. Beliefs that are irrational or aren’t based on the evidence aren’t knowledge.

The Gettier problem Justification is usually a matter of evidence, e.g. what I remember. Gettier: It is possible to have JTB without knowledge.

Infallibilism Since I can’t know something false, if I know that p, it is not possible that I could have made a mistake. Justified belief is true belief. If my belief could be false, then it is not justified. Is this too strong?

Development Condition 4: My justification for believing that p ‘stands up to the facts’. I know that p if my justification for believing that p is ‘undefeated’.

Objection Is this enough for knowledge? That’s Judy! That’s Judy! Case 1: Meeting Judy Case 2: Meeting Trudy Is this enough for knowledge? Or should we consider what the facts might have been?

Reliabilism S’s belief that p is justified if and only if S’s belief that p is produced by a reliable process Usually, this involves a causal connection between p and the belief that p. Reliabilism rejects the view that justification requires evidence. It is not the grounds that the person is able to produce that make a belief knowledge; it is the source of the belief.

Reliabilism and Gettier My memory of where I put my watch is reliable in normal situations. But this way of knowing where my watch is is not reliable in a Gettier situation. So I don’t have knowledge. What about Judy and Trudy? Even though recognising people is normally reliable, I’d still think Trudy was Judy. So I’m not reliable at recognising Judy, because she has a twin living locally; so even when I recognise Judy as Judy, I don’t know it’s Judy.

Reliabilism and context If the process that caused my belief can’t rule out real alternative possibilities, then it isn’t sufficiently reliable for my belief to count as knowledge (or even as justified). Reliability is relative to context.