"We've reached the place I told thee to expect,
Where thou shouldst see the miserable race,
Those who have lost the good of intellect."
Dante, Divine Comedy, The Inferno, Canto III
When Dante is led by Virgil through the gates of Hell, before he is ferried across the river Acheron by Charon, he enters the Vestibule of Hell where the futile abide. Dorothy Sayers explains in the commentary to the Penguin Classics edition:
"The vestibule is the abode of the weather-cock mind, the vague tolerance which will neither approve nor condemn, the cautious cowardice for which no decision is ever final. The spirits rush aimlessly after the aimlessly whirling banner, stung and goaded, as of old, by the thought that, in doing anything definite whatsoever, they are missing doing something else."
Virgil explains that these people have "lost the good of intellect". In the tradition of Aristotle and Aquinas, the good of intellect is truth. It is for this reason that they cannot commit to any deed or judgement - they have no language which is able to even make sense of the word truth, and they live in a relativistic world where everything is as good as anything else. Why bother making any final decisions in such a world?
But why is the good of the intellect truth? We need to understand what intellect and good mean in an Aristotelian sense to answer that.
For Aquinas the faculty of the intellect is something different from what we would call reason - reason has to do with inquiry and proof, while intellect relates to the direct mental grasp of truths. It has a direct moral function, in that it is able to apprehend goods that are presented to the will. It is distinguished in this sense from a merely speculative function, and has a practical element.
The good of the intellect is truth, because by apprehending that truth the intellect will move the will to act in certain ways. People will lose this faculty if the will is turned continually away from the higher intellectual level. If they are willing to use only the purely speculative or rational part of the intellect, and deny that there is a moral element to truth, they will live in absurdity, like those in the Vestibule. This is one of the inherent dangers of living in the Modern Age, which is dominated by the tyranny of relativism.
There is an excellent post on the intellect here