Reddit mentions of The Torch of Certainty

For many it may be not very possible in their current life, just as it's not possible for a cat, bird or dog to be enlightened. However, I do remember a sutta (unfortunately I don't know which one), where the Buddha taught a "simple" person a simple chant, which the repeated over and over and achieved enlightenment with -- kind of a precursor to mantra practice.

In the Tibetan tradition, if you've read the Torch of Certainty, or many other texts which teach about the preliminary practices, they all emphasize the precious human birth which is free from "the eight unfavorable states of existence in which undividuals are disadvantaged", one of which is:

> Mutes, said to be ignorant of the world of language, are dumb and do not turn their thoughts to the Dharma.

Which might not be the most tactful of wording, and I don't think it applies to people who cannot speak but are otherwise in full control of their faculties (eg., deaf people can use sign language, read and write, so they are not ignorant of the wold of language). I think it more applies to people so mentally disabled they're not able to understand language.

But anyways, even then it would just mean it's not easily possible to achieve enlightenment in their current life. They definitely can in future lives with better circumstances, and there are many things they can do to generate merit for that, even if they aren't completely aware of it (like being taken to holy places, having dharma read to them, circumambulating stupas and other relics, etc).