从六尘的领受或不领受来定义内识与外识，能领受六尘的识是识阴等六识，这六识都只是住在胜义根中，只能领受内六入，不能直接领受外六入，所以是只能住在五阴中的内识。但是从来不领受内六入的本识住胎识，却可以藉五色根来直接触受外六尘，所以名为外识；但这外识却不会了知及领受内、外六尘，只是藉外六入直接变生内六入给内识（六识）来领受，所以名为外识。
《阿含正义》第五辑
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The internal or external consciousness is defined by whether it can perceive or not perceive the six sense-objects. The six sense-consciousnesses, or consciousness skandhas, are the consciousnesses which can perceive the six sense-objects, and only reside in the inner supreme sense-organs; they can perceive only the internal six-entrances, not the external ones directly, and therefore they are the internal consciousnesses which can only reside in the five aggregates. However, never perceiving the internal six-entrances, the basic consciousness, or embryo-dwelling consciousness, can touch the external ones directly through the five sense-organs instead, and thus it is called the external consciousness. Nevertheless, the external consciousness can not understand and perceive the internal or external six-sense-objects, but only transforms the external six-entrances into the internal ones directly for the internal consciousness (six sense-consciousnesses) to perceive, so it is called the external consciousness accordingly.
The Correct Meanings of “The Agama Sutras”, Vol. 5
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