Alethic moods
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Some languages distinguish between alethic moods and non-alethic moods. Alethic moods indicate logical possibility, impossibility or necessity, whereas different moods are used to indicate other types of possibility/necessity. In such a language, "A circle can't be square", "can't be" would be expressed by an alethic mood, whereas for "He can't be that wealthy", "can't be" would be expressed by a non-alethic mood. As we can see, this is not a distinction drawn in English grammar.