Doubts. A thick, stubborn line of thoughts that circles around the same questions for which the only one capable of teaching the Warmaster Horus cannot provide an answer. A crushing sense of duty, grief for fallen and unavenged comrades, understanding of the enemy and sympathy for his fate, boundless adoration for the Emperor and his doctrines, the feeling of loneliness and not being understood, futile searching for truth and alternative paths, awareness that the goals set are unreachable, a sense of uselessness, jealousy and the desire to change, fix the established order of things…
Many centuries later, when even the very name of the Warmaster will be forgotten—when the warriors will draw great wisdom from ancient dusty tomes, when indomitable strength will be found in knowledge, and unwavering faith will be directed to the very one whom the godlike Horus called out to in his torment—then they will proclaim the truth that the first of the Primarchs either could not understand or did not want to understand: “Doubt is the essence of heresy.”