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Question in Short:
Why is it that the terms valuable and invaluable mean almost the same thing while the terms famous and infamous are almost semantically opposite in meaning? That is, one is used to describe something/someone well known for their good qualities, while the other is used to describe something/someone well known for their bad qualities?

Question in Detail:
One of the answers to the question titled Difference between “valuable” and “invaluable” [closed] suggest that the term valuable is almost synonymous with invaluable, although the preferred answer points out their subtle differences as being costly and priceless, respectively. That is, something that can be bought or sold (valuable), as opposed to something that cannot be bought or sold (invaluable), yet is treasured.

Why is it that the terms famous and infamous are not semantically synonymous according to these online references?

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  • possible duplicate of Difference between "valuable" and "invaluable" @Bill - I can't believe you actually used the ELU site search facility before asking. Variants of this one are always being asked. Jan 27 '14 at 4:09
  • I don't understand the question. The prefix in- means not, and meant not in Latin. Both famous and infamous are derived from Latin roots. And you are asking why famous and infamous have opposite meanings? This isn't unusual: consider discreet and indiscreet, noble and ignoble. Jan 27 '14 at 4:22
  • @Peter Shor: I suppose what bothers OP there is that famous and infamous both imply "well-known", either positively or negatively. The best we can do for an "antonym" is probably non-famous, but that's not exactly in common use. Jan 27 '14 at 4:31
  • I understand the Latin meaning of the 'in-' prefix meaning not, and I suppose to a certain degree it is correctly used to describe the difference between the terms 'famous' versus 'infamous'. I was simply using 'valuable' and 'invaluable' to describe a situation where it is not used in this context.
    – Bill
    Jan 27 '14 at 5:03
  • I suppose that currently in English, famous just means well-known and not well-known for good qualities (the word renowned better fits this definition). However, I expect that when the in- was attached to obtain infamis in Latin, it had a meaning closer to renowned. Jan 27 '14 at 5:09
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The prefix does its job faithfully regardless of the ultimate result of connotation or implication. Some explanation may be in order.

in-: prefix denoting 'not'.

in + famous → not famous
&
in + valuable → not valuable (Patience!)

fame: good reputation; famous: widely known for something good;
infamy bad reputation; infamous: widely known for something bad. – Naturally?

value (n): worth;
(to) value (v): to estimate the worth of;
valuable: that whose worth can be estimated;
invaluable: that whose worth can not be estimated; too valuable.

It all adds up nicely.

The prefix modifies in a mechanical way here, while the meaning on the other hand, depends on the nature of the word, its original implication, even its etymology and usage. Even not valuable can be understood in the right context to mean something that cannot be valued (not value+able).

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  • Can you apply this logic to 'flammable' and 'inflammable'? Use of the latter is discouraged by safety experts (sorry, no cite) because it can be mistaken for 'not flammable'.
    – Jim Mack
    Jan 27 '14 at 11:49
  • @JimMack That's a different in-, the preposition. Jan 27 '14 at 12:25
  • @JimMack The prefix in in inflammable is not the one we use for deriving an antonym. The in there implies the inherent nature of a material to burn, i.e., without need for an external source of fire such as a spark. Volatile liquids tend to burn all by themselves without external influence.
    – Kris
    Jan 27 '14 at 13:48
  • cf. inborn: from in "within" + boren "brought forth" (etymonline)
    – Kris
    Jan 27 '14 at 13:59

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