Matthew McVeagh | my conlangs
Writing without speech, but as if representing speech.
By which I mean a pasigraphy, which is a language which is only a writing system without any spoken version to represent, but not ideographic as most pasigraphy conceptions have been so far (Blissymbols; Leibniz' characteristica universalis).
Instead the symbols of the writing are structured as if they are representing segments of sound, just without actually doing so. The language would be structured grammatically like all others, including having 'morphemes' or something like them, but these would not (necessarily) be represented by one symbol each. Instead they would (mostly) be made up of sequences of symbols, just as alphabetic (/abugidic/abjadic) and syllabaric writing does. The symbols used would be from a much smaller list than the set of morphemes/words, and each would be used, arbitrarily, in many different morphemes with no semantic connection. The difference with phonographic writing would be that they simply do not represent any particular sounds.
Of course someone - the original creator or someone else - could assign sounds to the symbols. But no such assignation would be a 'correct' one. It might be possible to structure the distribution of symbols in a way analogous to phonotactic and morphophonemic patterns. That might make it easier to associate particular symbols with particular sounds but there still wouldn't be a 'correct' correspondence, only perhaps more or less pronounceable ones.
A further thought: imagine shaping the symbols to match a set of easily producible hand signs. You could then have a language with a duality of physical medium but still without speech: writing and signing.