Taking knowledge as a starting point › A Mental State That Reaches Into the World

Williamson treats knowing as a mental state. Its being factive means that knowing that something is the case entails its truth. If you know the report was submitted, it was submitted. Believing the report was submitted does not have that implication. Knowledge and belief therefore differ even when confidence feels the same from the inside.

His claim is stronger than the ordinary observation that knowledge requires truth. The proposed mental state itself involves the world being a certain way. A person who knows that the door is open and a deceived person who merely believes that it is open can differ mentally, in this broad sense, despite having indistinguishable experiences. This is an externalist proposal about mental states: what state you occupy can depend partly on circumstances beyond your skin. The publisher's summary identifies this extension of externalism as a central commitment.

Consider another hypothetical pair. One worker sees a genuine submission receipt. Another sees a perfect-looking forgery and has no reason to suspect it. Both may describe their experience identically. Williamson's proposal allows the first worker's knowing to matter to the explanation of what happens next. The second worker's convincing experience remains psychologically important, but it does not establish the same relation to the submitted report.

The point does not require calling the deceived worker foolish. An account of what someone knows differs from an assessment of whether they are blameworthy. Nor does it show that everything mental lies outside the head. It challenges the requirement that every genuine mental state be completely determined by what the person and their environment look like from a strictly internal perspective. Whether that challenge succeeds remains a philosophical question.