Every mental model you trust implicitly has a hidden load-bearing assumption — one that felt so obvious when you built the model that you never wrote it down. William James noticed something structurally similar when he distinguished between 'live' and 'dead' hypotheses in The Will to Believe (1897): a hypothesis is 'live' when it can still be doubted, and 'dead' when it's been absorbed so fully into habit that doubt no longer feels available. The danger isn't that your models are wrong. It's that the most consequential ones have quietly gone dead — you're no longer choosing them, they're choosing for you. The practical move is uncomfortable but specific: take one mental model you've applied so often it feels like perception rather than interpretation, and try to state the assumption it cannot function without. Not to discard the model — to make it live again.

What would change in how you operate today if your most trusted mental model turned out to rest on an assumption that was true ten years ago but is no longer?

Drawing from American Pragmatism — William James (The Will to Believe, 1897)