Consider the following two sentences:

  • Suppose that for a certain liquid the values of x1 and x2 are 9.762 and 3.91 receptively, but the true exact values are unknown.

  • Suppose that for a certain liquid the values of x1 and x2 are 9.762 and 3.91 receptively, but the true exact values are not known.

For me, I think the second sentence gives more sense than the first one. My goal is to express that the real values are as stated but we don't know that.

What is the best way to express it?

  • 3
    There is no difference in denotation. // 'Unknown' is further from the verby end of whatever continuum is involved than 'not known' is, so connotes (if it connotes anything) more strongly the unknown-ness of the values; 'not known' is closer to the verby end, and so connotes more strongly the scientists' ignorance of the facts. I'd say there is little to choose between them here, but I'd use 'not known'. Oct 20, 2017 at 9:01
  • 'Known' is the passive form of 'know', so still pretty verby. 'un-' can mean to undo, so perhaps 'unknown' carries a possible interpretation of 'forgotten'; but unlikely...
    – AmI
    Oct 20, 2017 at 20:05

2 Answers 2


"Unknown" means exactly the same thing as "not known".

However both terms can be ambiguous, in that they include both things of which the subject is unaware, as well as things of which they are aware but ignorant about.

This dichotomy was famously enunciated by one Donald Rumsfeldt when he spoke of "unknown unknowns" and "known unknowns".

  • So accordingly, which senstence of the above given ones best fit the expression?
    – Nizar
    Oct 20, 2017 at 8:26
  • @Nizar Really there is no difference, but Edwin's comment above is interesting.
    – WS2
    Oct 20, 2017 at 12:13

While both phrases/words have the same literal meaning, they can carry different implications.

To add to @EdwinAshworth's comment, I think that "not known" is more likely to carry an implication of "not known yet, by us, at this time". In other words, "unknown" can (or is more likely to) carry an implication of "fundamentally not knowable, or at least not known by anyone".

In a scientific context, I would reserve "unknown" for things that are not known by anyone, and use "not known" as a more simple statement of fact to describe a specific situation or measurement.

  • Sorry, I have to downvote, because I disagree there's a subtle difference in implication or situations where you'd use one or the other. Semantically, I think they're identical.
    – Dan Bron
    Dec 19, 2017 at 15:15

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