I'm trying to understand what counts objects as being members of a class. I'm getting stuck when there are variations for each object of a class but they might still be said to be part of that class.

For example; below there are 3 squares coloured in shades of orange. In the sample of squares, each shade is slightly different to each other and the colour of "orange" itself.

Is it correct to claim that "All squares (in the sample) are orange" even though they are different shades of orange?

enter image description here

And if it is correct to claim that "All squares (in the sample) are orange", what determines how similar the shades must be to orange so that the claim is true?

  • It is an empirical question: do you have defined "orange" in terms of a certain interval of wavelength? Sep 25, 2022 at 9:42
  • @MauroAllegranze no I mean orange to be an attribute which the term squares (in the sample) connotes. Another example is having three objects of different sizes and calling them 'medium sized objects'. Sep 25, 2022 at 9:55
  • If they are similar, they are so for some aspect: that aspect is the "common core" that applies to all. Sep 25, 2022 at 11:16
  • @MauroALLEGRANZA: That's definitional, not empirical. Empirically, they can only reflect back wavelengths shone on to them. Artificial lighting might generate different categories to being illumine by the sun.
    – CriglCragl
    Oct 3, 2022 at 15:08

2 Answers 2


A lot of time classification is thought of as relative to a set of background interests, aims, other intended comparisons, etc. Abstracting from your particular example of color, a different way of interpreting the general question is to focus on those special classificatory categories which presuppose something like "absolute sameness"? The members of the intended class can differ with respect to other qualities but not the qualities definitive of the class itself? Then, what you would be looking for is something like formal, logical categories such as the category of an object, property, etc. Precisely because these are not real qualitative categories, the members of the class can differ in many other respects, as long as they don't differ with respect to the formal, logical category. In essence, it really depends on the types of classes one is focusing on.


Color poses a problem, since we have to distinguish physical color - that is simply certain wavelength, and perceived color - that is the color seen by a human, after it is processed by the brain (specifically, all the squares in the OP are really combinations of red, green, and blue light emitted by our monitors, whose different combinations of intensity make them look as different shades of orange.)

In case of physical color, scientists and/or engineers would typically adopt a convention that a range from such to such wavelength is a "color" see, e.g., electromagnetic spectrum), which means that this range would contain a continuum of possible colors (unless wavelengths are discretized for some reason).

In case of perceived color the color would be decided by consensus of humans. This is subjective as we can see from different number of basic colors existing in different languages - ranging from 2 in Bassa and Pirana languages to 12 in Italian, Russian and Hebrew (English has 11).

A closely related phenomenon is phonological system - that is how all the possible sounds that a human mouth can produce are split into groups that, for the purposes of a given language, interpreted as a single sound.

Another notorious example of different color perception is illustrated below (borrowed from this page): enter image description here

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