Gnosticism is logical.
In fact, it can be argued that logic and rational conclusions are the gnostic's gods.
The gnostic uses rationality to construct abstract ideas of how the world works . . . of what full reality is. The gnostic then embodies these abstract ideas into structures (the belief in the abstract as embodied in structure is the gnostic's perverse parallel of the Christian's idea of the sacramental). The structures and their parts then become the dogmas and doctrines of the gnostic.
The problem is, it's always a closed-off system.
It is closed off from the Tao.
The Tao is nameless. The Tao is undefinable. The Tao isn't empirically provable and can't be quantified or nailed down or even used.
The Tao defies, stands outside of, the gnostic's neat, tidy, and logical structures.
The Tao, therefore, quietly and triumphantly defies the gnostic, which infuriates the gnostic. There's a reason Lenin strived hard to shut down the Russian Orthodox Church: by asserting the Tao, it asserted an element of reality that stood outside the Marxist structure. The Tao simply doesn't make sense within the gnostic's system, and because it doesn't make sense to the gnostic, it must be denied . . . or suppressed or stomped out.
The right hemisphere of the brain is what connects to the Tao. The right hemisphere appreciates what it can't know and is charmed by what it can't name. It doesn't need to quantify everything, realizes that many things transcend empirical verification, and appreciates things (like, say, flowers and poetry and love) that aren't useful.
Such things frustrate the hell out of the left hemisphere and, not coincidentally, the gnostic. The gnostic is driven by his left hemisphere. The more left-hemispheric the gnostic, the more extreme the gnostic.
In a sense, someone might respond, "This is tautological. You say the gnostic is left-hemispheric because the left hemisphere drives the gnostic."
Definitions are tautological. When I say "The gnostic is a left-hemispheric thinker who takes his ideas to religious levels," I am positing the left hemisphere as part of the definition. That's what definitions do.
But more important, gnosticism and its left hemisphere understanding of reality are, like a tautology, in a closed-off system. Like a maniac in his mental maze, the gnostic is trapped within his left hemispheric reality. It's tautological, but so are most forms of insanity.