Reader : please consider that I don't know anything about biology, which is the case by the way.

Context : I am currently preparing a presentation for a research grant application, which I will submit to a jury of mathematicians for their decision. My first slide aims to highlight the fact that human beings have a natural reflex to classify the objects around them, and to do this I consider the case of two objects that are a priori distinct, but which are equivalent by convention: they are just two apples (image). To be clear, I need to say why they are equivalent.

Question : Biologically speaking, what justifies that these two a priori distinct objects are indeed two apples, and therefore biologically equivalent? As a biologist, what is the justification you would give to say these two things are quite the same?

Two distinct apples :)

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    $\begingroup$ Well, personally I wouldn't. The distinctions made in biology go down to the molecular level, we differentiate between the colour red and the colour green on a macro and micro levels. Also on the differing effect the colour has on a species that likes apples - not just humans, that is we'd be looking at the item in context of the environment too. Then there's the evolution of how the differences came about .... it goes on. Doesn't this more come down to a Philosophy question than a biology one? $\endgroup$ Aug 31, 2022 at 14:22
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    $\begingroup$ I’m voting to close this question because it is about ontology and the philosophy of classification. Replace the apples with any two similar-but-distinct objects and the question remains the same. Perhaps there is an on-topic interpretation of this question regarding taxonomy, but it's not clear to me. $\endgroup$
    – acvill
    Aug 31, 2022 at 17:05
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    $\begingroup$ On reflection, you might want to take a look at our Psychology & Neuroscience stack. If asking there, you'd need to be sure to frame your question in existing models of perception and language. $\endgroup$ Aug 31, 2022 at 17:18
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    $\begingroup$ I agree this is off topic. However, you got some answers anyway before being closed that you seem to indicate appreciating and wanting to employ in your presentation. Just a reminder, then, that I think you should plan to be citing these users as your original source(s) for these ideas (especially since the answers you got are less biological fact and more ontological discussion). $\endgroup$ Sep 1, 2022 at 11:02
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    $\begingroup$ How do you know they are not snapples? $\endgroup$
    – user338907
    Sep 1, 2022 at 11:35

3 Answers 3


Biologists aren't the people who decided what an "apple" is, the general population of users of language are.

You could try back-create a definition that is biologically relevant based on descent, for example "fruit of plants in the genus Malus", but that would include a lot of apples that aren't really apples the way English speakers in general use the word.

You could limit it to just fruits of Malus domestica, but that probably wouldn't work great, either, because there are several fruits outside that species that a typical English speaker would still call "apple".

A relevant botanical definition is that an apple is a pome:

A pome is an accessory fruit composed of one or more carpels surrounded by accessory tissue. The accessory tissue is interpreted by some specialists as an extension of the receptacle and is then referred to as "fruit cortex",[2] and by others as a fused hypanthium (floral cup).[2]

...but many botanical fruits of this type are not "apples" in the English use of the word, such as pears or hawthorn.

It happens a lot in language that biology and culinary or other language issues conflict, for example the "is a tomato a fruit?" debate. There are also other popular language classification quirks that people debate on the internet like "is a hotdog a sandwich?". These are all questions about language and how language is used, not about any biological or other scientific essence of things.

Biologically speaking, your brain learning these things is a lot like how an artificial neural network is trained to, say, identify road signs. Just like when you complete "Captcha" tasks to "prove" you are a human when acting online like "select all images with a stop sign in them", and this training data is sent to an AI algorithm, as a developing human the adults around you will hand you things and say "apple". You then assign a little slice of all the features you perceive and associate those with the concept "apple" in your brain. They're also correct you and say "no, that's an orange" if you expand your definition beyond "apple", and you'll learn to break associations between those orange-specific perceptions. In this way, the things you call "apple" are just the things that other people call apple, and that's how we can use language to communicate with each other: as long as we all mean roughly the same thing when we say "apple", it's a useful concept, even if it doesn't make sense biologically.

  • $\begingroup$ Thanks a lot for your answer. I accepted this one because it answers my question perfectly ! $\endgroup$
    – Baloo
    Sep 1, 2022 at 8:08

I think one thing you are overlooking is that humans employ hierarchies of classification. We group things at multiple levels depending on the degree and types of similarities. In the picture you've provided the two images are the same in the sense that they are both examples of the genus Malus:

The fruit is a globose pome, varying in size from 1–4 cm (1⁄2–1+1⁄2 in) in diameter in most of the wild species, to 6 cm (2+1⁄4 in) in M. sylvestris sieversii, 8 cm (3 in) in M. domestica, and even larger in certain cultivated orchard apples. The centre of the fruit contains five carpels arranged star-like, each containing one or two seeds.

They are different in that one is perhaps an example of the variety Delicious, and the other is perhaps of the variety Granny Smith. The two varieties differ in skin color and taste among other attributes, but share many of the attributes associated with the broader category of genus Malus.

They are also different in the most abstract sense because they are two different objects.

Which classification is relevant depends on context. If I'm making a grocery list the variety is the most significant classification. If I'm writing a fairy story the genus may be all that's relevant.

  • $\begingroup$ Your answer is very clear and helped me to think about what I should say to introduce this topic. Thanks :) $\endgroup$
    – Baloo
    Sep 1, 2022 at 8:06

My answer: based on DNA similarities I can confidently tell you whether two "things" are so similar they merit similar names.

Other points to mention: We might call two things similarly without necessarily implying that they are the same thing (e.g., a giant panda and a red panda). Both of those pictures are apples because they look, taste, and feel like an average apple and we decided on that before we could check their DNA. There's a Stanford lecture on Human Behavioral Biology that discusses how we humans tend to categorize objects into groups to ease information processing that you might want to check out.


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