Coherent, yet false?
On the axis: Justification of belief · a coherent system
A set of beliefs can be perfectly coherent and yet wholly cut off from reality: a well-built tale, a systematic delusion, a conspiracy theory that explains everything and absorbs every objection. If coherence is not enough to touch the world, must beliefs not also answer to something outside, to the experience that contradicts them?
Justification
If all knowledge of the world comes from experience, there must be that are not inferred: Russell calls them hard data, what resists the utmost effort of doubt, and out of them he builds everything else by : objects, other minds, the past. The difficulty is that these data are private and momentary, so that the public world has to be rebuilt at great cost. Russell admits this and retreats: in 1912 he held the data to be indubitable, whereas (1948) grants them only a probable base, weakening the foundation by that much.
Our Knowledge of the External World (1914), lectures III-IV · Human Knowledge (1948), part II
Anchors: Russell · Schlick