The intellect is a faculty that allows us to know the physical world and a little of the psychic world, but no more, and as such, it is very limited. The intellect alone cannot know the truth.
To know the truth of a rose is not just a question of perceiving its shape, colour, and scent. The truth of a rose is its soul, its emanation, its existence, which the intellect cannot grasp.
To know a rose, we must penetrate all the elements that make it a rose.
The same is true of a human being; the truth about someone encompasses everything about them – their soul and spirit, their thoughts and feelings, their plans for the future and so on.
As long as you do not know all these things, you do not know the truth about a person.
You know a little bit about them, their appearance, but not the truth. The truth is a synthesis, which can only be grasped by the spirit.