# Does many valued logic hold in unambiguous cases?

My understanding of many valued logic is that it is used in cases where an aspect of a proposition is vague or not well defined. The example Wikipedia gives is:

'This apple is red.' Upon observation, the apple is an undetermined color between yellow and red, or it is motled both colors. Thus the color falls into neither category " red " nor " yellow ", but these are the only categories available to us as we sort the apples. We might say it is "50% red". This could be rephrased: it is 50% true that the apple is red. Therefore, P is 50% true, and 50% false. " https://en.wikipedia.org/wiki/Principle_of_bivalence#Vagueness

It seems to me that this is only the case if 'red' is not well defined. If it were well defined as, for example, 'such that at least 90% of the apple's surface area were to reflect light at a wavelength of 650nm (give or take 25nm) when viewed under white light' then it would seem that there is, in fact, a sharp line between true and false, such that the proposition, 'the apple is red', is true if and only if the apple fits the criteria given in the definition of 'red'. This would seem to me to indicate that many valued logics are not useful in cases where a proposition is well defined, however, people like Graham Priest seem to hold the position that many valued logics can hold even in well defined cases, i.e., 'some real contradictions are true'.

My question is, is it true that many valued logics can hold in unambiguous cases, and if so, how?

• There's also Zadeh's fuzzy logic en.wikipedia.org/wiki/Fuzzy_logic where truth values can take on any real value between 0.0 and 1.0 inclusive. So your 50%,90%,etc red are easily and quantitatively accommodated. – John Forkosh May 11 '17 at 23:05