How explain conscience if the soul is a mere life-principle? To be alive is not the same as to have a conscience. There are plenty of living things that do not and cannot have a conscience, and there may be dead souls with a conscience. But even if the property of being alive and the property of having a conscience were co-extensional they would not be the same property.
Each living thing has its own life that it naturally affirms and protects. Although life is ceaseless self-assertion and self-protection, conscience may demand self-denial to the point of sacrificing one’s life for others or for one’s cause. A curious life-principle it would be that would preside over the death of the soul-body composite of which it is the animating principle.
Conscience reveals the discrepancy between what I am and what I ought to be. The awareness of this gap and its concomitants such as regret, remorse, desire to do better, and so on, is not reasonably ascribed to a mere life-principle. Thought, too, like moral evaluation, goes well beyond what a mere life-principle could deliver.
Animation, cogitation, evaluation. A soul worth its salt ought to be adept at all three. Whether one and the same item or ‘principle’ can fill the bill is a topic that needs careful thought with deep attention to the researches of the twin titans of our tradition.