Aristotle famously maintains in his treatise On the Soul that there are three kinds of living things-- plants, animals, and humans--and he speaks of each kind as having a soul. St. Thomas Aquinas adopts these same views. Nowadays, however, many thinkers reject them on the grounds that they are incompatible with modern science. Molecular biology is seen as affording a superior way of categorizing life forms. Evolutionary biology appears to have established that humans are simply primates with adaptations somewhat different from other primates. Plant physiology, uncovering plants' directional responses to a wide variety of different stimuli, provides reason to think that plants sense light, pressure, odor, pathogens, etc., in which case plants are simply non-mobile animals, similar to coral. Neuroscience, by showing the dependency of thought, choice, perception, and emotion on the central nervous system, especially on the brain, seems to have eliminated any need to posit a soul.

Plants, Animals, People, Aliens addresses these and a variety of other science-based claims about living things by bringing Aristotelian-Thomistic teachings to bear upon them. While not a systematic work, it both treats some of the most hotly debated philosophical questions concerning living things, such as what it means to be alive, as well as treating questions less often raised by philosophers, such as whether animals have a theory of mind. Though geared primarily to those conversant with the Aristotelian-Thomistic tradition, many of its key concepts are traced back to experiences that we all share in common, thus rendering these concepts intelligible, if not convincing, to those with scientific and other backgrounds who are curious about this philosophical approach to living things. The author strives to show how Aristotelian-Thomistic philosophy and science complement one another by sketching out the fuller understanding of the living realm that the two together provide.

The book's final section considers the possibility and the likelihood that embodied rational beings exist elsewhere in our universe. Here, theological arguments drawn from the Christian faith are presented alongside philosophical and scientific arguments. This section closes with a theological discussion of the disputed question of whether extraterrestrials, were they to exist, should be baptized.