Reworded as requested.

I am confused about when one uses the term “incompatible” and when “compatible”.

Is it sometimes believed that compatibility is about “working” and incompatibility about “not working”. However, consider the following two statements:

  • Printer X is incompatible with the latest version of windows –does not work
  • Grenade X is incompatible with normal handling – works (detonates)

Apparently incompatible can mean both not to work and to work”. Perhaps it is about something being “suitable”?

  • Vending machine X is incompatible with the new currency – does not work, not suitable
  • Vending machine X is incompatible with foreign coins – does not work, suitable

It seems that incompatibility can mean to be not suitable and suitable!

So, is there an easy way of explaining when to use the word compatible and when incompatible?

closed as unclear what you're asking by RyeɃreḁd, MetaEd, aedia λ, tchrist, Andrew Leach Apr 29 '14 at 22:13

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  • For first assumption is incorrect, as saying an explosive is compatible with salt water implies that it works in salt water. Incompatible is the proper word there, and there is no paradox. – Oldcat Apr 22 '14 at 21:02
  • @Oldcat, I tend to agree with you and would much rather use incompatibility with salt water to mean NOT detonate and actually have been using this definition until it was brought to my attention that the opposite (compatible) was more frequently used! – Richard Johnson Apr 22 '14 at 21:17
  • Actually, I think there is the seed of a good question here, and urge you to reword it. But the real problem is with work not compatible; consider that "it does not work, as required" is the opposite of "it does not work as required", though a hasty reader will take the first to be the second. Consider also an explosive that is rendered inoperable by salt: it is in fact incompatible with salt water, but your example would call it compatible. – TimLymington May 9 '14 at 10:51
  • TimLymington, I did not realise the ambiguity! Your point about explosive is well made. I think I am getting to an answer that is satisfactory to me: – Richard Johnson May 9 '14 at 21:48
  • TimLymington, Your point about explosive is well made and shows that one must be clear about what is incompatible with what and not leave anything unsaid. Everything becomes clear if one forces oneself to only express the incompatibility of two propositions that can each be true or false, i.e. [operation of explosive] is incompatible with [use near salt] means that both cannot coexist, i.e. both be true. Hence [use near salt] will not [operate the explosive]. This aligns with the logical definition of incompatibility. – Richard Johnson May 9 '14 at 21:56

Your "explosive" example is poorly expressed.

My explosive is compatible with salt water

means that your explosive performs as you require it to in the presence of salt water. In this case, the required performance is "do not explode until given a specific signal".

If your explosive, when dumped into salt water, does not explode until given a specific signal, then it is compatible with salt water. But if it might explode on its own, then it is incompatible with salt water, because it does not perform in the required fashion.

The difference there is between "I require it to NOT DO X, and it succeeds" vs. "I require it to DO X, and it fails". If the item does what you require it to do under the circumstances given, then it is compatible with those circumstances.

So, you could express it better by saying

"my explosive is compatible with salt water" means that it works as required (by not exploding).

  • Thanks @Hellion. Whilst making things rather more precise, I still have the issue that I require a) the explosive NOT DO "explode" and vending machine to NOT DO "operate", but the first I say is compatible with water and the latter is incompatible with foreign currency. – Richard Johnson Apr 22 '14 at 21:22
  • No, you require that a) the explosive perform the action "not explode" (which it does), and that b) the vending machine perform the action "operate" (which it does not). – Hellion Apr 22 '14 at 21:36
  • Hmmmm. I agree with your point a) I.e require explosive not to explode with salt water (only explode given the right stimulus). But I thought that I also require the vending machine NOT to operate with foreign currency (only operate on English). Yet your sentence b) says different. I'm either missing something here or expressed myself ambiguously in the first place. – Richard Johnson Apr 22 '14 at 21:58

I might have worked out AN answer. Yes, it all depends on suitability (whether action or no action), but it matters to whom it is suitable. When an adversary is involved, who wishes to break into a system, then this introduces a negative suitability....i.e.

  • For an explosive with salt water, it is the explosive that does the action and if "suitable" means NO action to the user of the explosive then this state of affairs is "compatible".

  • For a vending machine with foreign currency, it is the currency that does the action and if "suitable" means ACTION to the thief then this state of affairs is "compatible", so NO action would be "incompatible".

Anyway, thanks for the help stimulating some lateral thinking.

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