Seneca, Epistle 71.27:

Non educo sapientem ex hominum numero nec dolores ab illo sicut ab aliqua rupe nullum sensum admittente summoveo. Memini ex duabus illum partibus esse compositum: altera est inrationalis, haec mordetur, uritur, dolet; altera rationalis, haec inconcussas opiniones habet, intrepida est et indomita. In hac positum est summum illud hominis bonum. Antequam impleatur, incerta mentis volutatio est; cum vero perfectum est, inmota illi stabilitas est.

My translation:

I don’t remove the wiseman from the number of men nor do I take away pains from him as if from some rock that lets in no sensation. I keep in mind that he is composed of two parts: the one is irrational – this is bitten, burned, feels pain; the other is rational – this possesses opinions unshaken, it is unafraid and unconquered. In this is placed that highest good of humans. Before it is filled, the mind has an unfixed rolling about; but when it is complete, it has an unmoved stability.

–––

The mind that exclusively uses the irrational unstable egoic control system suffers pain in the altered state, it is shaken, it is afraid, it is conquered. The mind that uses the deterministic control system, the perfected, filled mind, is unshaken, unafraid, unconquered, it possesses an unmoved stability.

This passage is part of a larger argument that while the wiseman suffers harm, he is not disturbed.

The irrational part is the egoic system of control. The rational part is the deterministic/transcendent system of control. The mind first uses the egoic system of control exclusively. This is characterized by irrationality and instability, the unfixed rolling about of my translation (Seneca’s incerta … volutatio). Seneca describes the egoic system of control as if it were the body – the body feels pain. Seneca allows the wiseman to feel pain in the body, i.e. the egoic control system understood. He denies that the wiseman is disturbed by that pain in the mind, i.e. the deterministic control system.

When reading ancient philosophy, understand [body = egoic control system] and [mind = deterministic/transcendent control system].