The greatest enemy of knowledge is not ignorance; it is the illusion of knowledge.

Not knowing is a fairly harmless state, because an empty space at least invites filling. Feeling certain while being wrong is far more damaging, because it closes the door on learning. Ignorance can be cured by curiosity, but the illusion of knowledge passes itself off as understanding, so it is never examined. Someone who knows they don't know will ask and check, while someone who believes they already know will not.

The thought fits the scientific tradition Hawking belonged to, where theories are provisional and progress comes from admitting where they fail. It also echoes Socrates, who located his wisdom in recognizing the limits of what he knew. One caution: the line is widely credited to Hawking, but the attribution is disputed, and similar wording is often linked to the historian Daniel Boorstin.

Today you might apply it when a health claim in a headline feels obviously true. Before repeating it to a friend, ask whether you could explain where the evidence comes from. If you can't, what felt like knowledge may only be familiarity, and that is a good moment to look further.