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Why are the aspirated and unaspirated English /p/ sounds considered allophones of one phoneme?

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Diagram of a human head in profile showing the location of vocal organs and possible places of articulation for speech sounds, with labels for lips, teeth, tongue, palate, uvula, pharynx, and vocal cords, and numbered regions corresponding to types of articulation such as bilabial, alveolar, and velar.
phonetics Diagram depicting the location of human vocal organs and possible places of articulation used for speech.

In English linguistics, the aspirated [pʰ] and unaspirated [p] sounds are considered allophones of the same phoneme, /p/, because they are in complementary distribution and do not create a minimal contrast that distinguishes word meanings.1

These sounds are contextually determined variants:1
* Aspiration: The aspirated [pʰ] typically occurs at the beginning of a word, such as in "pin".2
* Unaspiration: The unaspirated [p] occurs in specific environments, such as after the /s/ sound, as in "spin".2

Because these variants never occur in the same phonetic environment to change the meaning of a word, they are not separate phonemes. While they are phonetically distinct, English speakers perceive them as the same underlying unit. This relationship is governed by phonological rules that predict which allophone will appear based on the surrounding sounds. In contrast, languages like Thai treat these as separate phonemes because the difference in aspiration can change a word's meaning.1342