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. 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. Plant physiology provides reason to think that plants sense light, pressure, odor, pathogens, etc. Neuroscience 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. The author strives to show how Aristotelian-Thomistic philosophy and science complement one another by sketching out the fuller understanding of the living realm.

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