The Aristotelian/ Thomistic conception of the universe, based upon observational (sensational) experience, divides all material things up into four forms: (1) Inanimate things, (2) Vegetative life, (3) Sensible Life, and (4) Intellectual, or Rational, life.
Under the first grouping, inanimate things, we find things like rocks, planets, stars, mud, etc. The things grouped under this first level, show no signs of life, not even the most basic characteristics.
The last three divisions describe living things. Those living things which fall under the higher divisions also possess the qualities of the lower divisions, though in a different way. However, those living things in the lower divisions do not appear to possess the qualities of the higher divisions.
The first, and lowest, division of living things is known as vegetative life. Vegetative life, as the lowest division of living things, is common to all living things. It describes the basic functions of life, things that are done entirely unconsciously, namely, growth, nutrition, reproduction, and death. All living things demonstrate these qualities, but some, such as plants and viruses, do not demonstrate more than these characteristics.
The second division of life is sensible life. Sensible life is that level that describes all those forms of life that, in one way or another, gain sensible knowledge of their surroundings. Such knowledge may be gained through touch, sound, sight, taste, smell, any combination of these senses, all of these senses together, or any combination of these senses along with other senses that we have not mentioned. Some animals have special senses that give them knowledge of their surroundings, for example, the snake uses a form of heat detection to find its prey, and bats use some form of echolocation to find their prey, and to fly in the dark.
This second division includes any life form that is able to gain knowledge about its surroundings through some sense, and is able to react to the knowledge gained. This division of life demonstrates a certain level of consciousness and desires. All forms of animal life fit under this second division, possessing both vegetative and sensible life.
The third division, intellectual, or rational, life, as applied to material beings, seems to be predicable only of humans. This third division describes mans capacity for rationality. Human people clearly possess all three divisions of life, and, as such, would be considered the highest division of material life. It is due to this rationality that humans have the ability to construct and communicate with symbols that possess meaning - that is, language. It seems that rationality is necessary for a being to be able to give a sense, or meaning, to events, symbols, things, etc.
These divisions are discovered through experiential, or sensible, observation, which is the humans first source of knowledge. These divisions are discovered through the observations of natural philosophy, which is, today, known by the general term - the natural sciences. As such, it is important to note, that Naturalism, and even Materialism, is able to agree with everything that we have just said about the natural world. Certain philosophers, or scientists, may wish to explain the different divisions differently, and they may wish to give varying explanations of what life is, what reason is, etc., but they would agree with the general observations that we have just made.
In the Aristotelian tradition we refer to these forms of life as "souls". That is, the vegetative soul, the animal soul, and the rational soul. The term "soul" is an unfortunate term because it carries with it, in today's common usage, a certain definition that was not part of the Aristotelian definitions. In Greek, the term "psuche", which is translated into English by the term "soul", literally means, "breathe of life", or, quite simply, "life". (see: Perschbacher, Wesley J., The New Analytical Greek Lexicon (1990; repr., Peabody, MA: Hendrickson Publishers, 1995), 444.) So defined most people would quite willingly agree that plants and animals have "souls", because they are "living". However, quite frequently, when people speak of "souls", they understand this word in the Cartesian sense, that is, "Descartes argued that only persons had souls ... As the subject of though, memory, emotion, desire, and action, the soul has been supposed to be an entity that makes self-consciousness possible..." (William E. Mann, "Soul," in The Cambridge Dictionary of Philosophy, 2nd ed., ed. Robert Audi (1999; repr., New York: Camridge University Press, 2009), 866.) As this is primarily a question of definition, we will maintain the Aristotelian tradition, and continue to use the term soul to refer to life. We will then be able to distinguish the different souls, that is to say, the different life forms - the vegetative soul, the animal, or sensible, soul, and the rational soul. (Descartes definition of a soul is roughly the equivalent of the rational soul.)
We should also note, though we will look into it in more detail in a later post, that the term soul, is roughly the equivalent of the term "form" when applied to living things. That is to say, the soul is the form of the living being.
George P. Klubertanz, The Philosophy of Human Nature (New York: Appleton-Century-Crofts, 1953), 24. Cf. Aristotle, On the Soul.
There may be some further intricacies, but we are not concerned, at this point, with an in-depth description of all of the elements of each level.