In comparing animals and humans, or as put more modernly, comparing humans with other animals, the questions of the existence, nature, and necessity of a human soul often come up.
The question of existence of the human soul may be explored from different perspectives. For example, this question may be posed as "is there a human soul?" If so, what is it? Why do we need a soul? Why not just the brain? Is it needed to explain something, such as "continuation of life after physical death?" If so, is it just a contrivance to answer such peripheral questions?
To attempt a rational treatment of these questions, a two-phase approach is adopted: first, a rational foundation is laid out, and second, the principles established as part of this rational foundation are applied to specific questions, such as those posed above. The overall argument is the result of a modern treatment and integration of several diverse concepts Proclaimed by Bahá'u'lláh and explained by `Abdu'l-Bahá. These diverse concepts are presented aided by various quotes from the Bahá'í sacred Writings.
In the first phase, science and rationality are compared and their relationship established. A principle of primacy of logic and rationality is developed that includes science as a subset. Relationships are categorized into two broad classes: material and rational. A model of thought, human or otherwise, is developed to precisely define what it means to think. The central role of analogy to cognitive activities is described. The physical world is defined as a realm where the fundamental dimensions of time and space govern over all that is contained therein. Further, a number of realms of existence, or "Kingdoms," including mineral, plant, animal, and human Kingdoms are described.
In the second phase, a part scientific and part rational methodology is employed to specifically answer the direct and indirect questions about the existence of the human soul. It is argued that no physical system, including the human brain, can comprehend rational relationships because the rational and material realms do not overlap. This aspect differentiates man from animal. One aspect of spirituality is described as the ability to comprehend rational relationship. However, since computers clearly represent and process rational relationships, additional explanations are appropriate, and indeed, required. Comprehension of rational relationships is not the same as their representation or processing. Comprehension of an abstract rational relationship is realized only if it is analogically related and successfully applied to a different domain from the one in which it was discovered, when the two domains are materially disjoint.
It is concluded that the mere fact of comprehension of abstract rational relationships necessitates the existence and assistance of a non-physical entity, the human soul, which provides the power of rational comprehension from outside the physical realm. The Kingdoms of existence further explain how the human soul fits in a unified world of God.