Root meaning: Sound or sane thinking. Logical thinking based on true premises, on solid reasons.
Conventional: The proper exercise of the human intellect.
ACIM: The Holy Spirit’s thinking, logical thinking based on God’s premises. Sane thinking by which illusions are judged as illusory and sin is reinterpreted as a mistake. The means of salvation (see T-22.III.3:1); the opposite of insanity. The ego, being insane, has no reason. It applies logic to insane premises and thus is logical but totally unreasonable. Since the conscious mind is the domain of the ego (see consciousness), left to its own devices the intellect is completely irrational (see T-21.V.4). Our thinking only becomes rational when it is not generated by our conscious mind, but is inspired by our right mind, the home of the Holy Spirit. In other words, the Holy Spirit is not arational or non-intellectual, but quite the reverse. He actually applies the rules of sound thinking far better than we do. Reason is not an ability invented by the ego and reinterpreted by the Holy Spirit, for it is fundamentally antithetical to the ego. See T-21.V,VI, T-22.III.