The quality or state of being external; the degree to which something is outside, visible, or not internal.
From external (Latin externus) + -ness (Old English suffix creating abstract nouns from adjectives). The suffix became productive in Middle English for measuring qualities or conditions.
The concept of 'externalness' actually troubled medieval philosophers—they debated whether physical things we see really exist externally or if our mind creates that appearance, a question that echoes in modern neuroscience about perception.
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