The act of sticking firmly to a belief, rule, or principle; faithful observance or attachment to something.
From Latin 'adhaerere' meaning 'to stick to', composed of 'ad-' (to) and 'haerere' (to stick). The word originally had a physical meaning of sticking or clinging to something, which metaphorically extended to mean faithfully following rules, beliefs, or principles.
The physical root of 'adherence' (sticking to something) perfectly captures the mental concept - when you have adherence to principles, your mind 'sticks' to them like glue! This is why we also say someone is 'stuck in their ways' or ideas 'stick with us'.
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