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The following two sentences quoted from this scientific paper (emphasize added) actually mean the same thing: at any moment.

For such plants this is related to the choke/valve openings, compressor, and pump settings at every instance of time. These are the control elements.


The idea is to operate the plant, at every instant of time, as near optimum as possible.

Are "instance of time" and "instant of time" interchangeable?

If not, which one should be used? The phrases often occur in Physics textbooks with the same meaning.

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An instance is a particular example of something. This is an instance of an answer on EL&U.

An instant is a moment, an infinitesimal point in time.

Now, every instant is indeed an instance of time, because every infinitesimal point in time is an example of a time. There's a subtle difference, but it amounts to the same thing in some cases.

The overlap between the two is very small though.

It would be unusual to think of it this way though, unless perhaps you were someone who often thought about different examples of things moving through time and how they were alike or different from each other. In other words, unless you were a physicist.

It's much more idiomatic to say "instant of time" or simply "instant" when you mean such a point in time. I would recommend it.

If however you were taking several readings or measurements of something, then you would have several instances, and in the case of physics these might very well be of time. Here "instance of time" is the only correct one. Which is probably what leads these physicists to say "instance of time" when the rest of us would more likely say "instant of time".

  • The two terms are rather unrelated, as you rightly observe: "The overlap between the two is very small" -- they're not interchangeable. – Kris Jan 7 '15 at 14:24
  • E.g., "Thus when mathematics and physics postulate a particular instance of time in their grand project to calculate a reality based on time-space relation, they must not contradict themselves by assuming this instance of time to be of determinable ..." GoogleBooks books.google.co.in/… (S. K. Leun, Nature of the Self: A Philosophy on Human Nature ) – Kris Jan 7 '15 at 14:27
  • @Kris small but not non-existent. In the question the first sentence is unusual, but not incorrect. – Jon Hanna Jan 7 '15 at 14:47

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