The Human Person, Rational, Irreplaceable
The Human Person, Rational, Irreplaceable

The Human Person, Rational, Irreplaceable

Robert McNamara continues his exploration of what it means to be human


Achieving a deeper understanding the human person is becoming increasingly important in a culture that no longer recognises the inviolable dignity of every human being. Understanding the objective good that the human person is, through better understanding what the human person is—through his essential definition—we are better capacitated to act rightly in all our dealings with the human person, and so and so accord him his full and proper dignity.

The definition of the person can be for us a means by which to understand the reality, a lens through which to see the human individual more clearly and more completely. In this way, we can fix our gaze upon the human being, and have that gaze clarified, expanded and enriched. Such a transformation of our look is necessary if we are to have a transformation of action, and so come to a mature love of the human person.

Defining what it means to be a person

St. Boëthius, in early medieval times, first settled on a definition for the person, arguing that there are three essential features of the person as such (whether human, angelic, or divine), that of substantiality, individuality, and rationality, and thus concludes that the person is an "individual substance of a rational nature."

This definition became classical for Christianity, and is the hinge around which all later Christian thought on the person rotates. This is especially so during the late medieval period and in the thinking of St. Thomas Aquinas. Aquinas’ most sustained treatment of the person appears in a theological treatise, the Summa theologiae, and here as elsewhere he treats of the person almost exclusively according to its divine signification, according to its use with regard to the Individuals of the Trinity. Yet, despite this fundamentally theological mode of approach, there is much in the Summa that is likewise applicable to the person in its use regarding the human individual, and also according to a purely philosophical understanding. And so, almost eight-hundred years after Boëthius, Aquinas also defines the person in the very same way: "an individual substance of a rational nature." Yet, unlike Boëthius, he clarifies the meaning of each term in the definition with great precision, and thereby furnishes an enriched understanding of the definition. To reach a deeper understanding of the definition, it is best first to look at each of the component parts individually, that of: 1) substantiality; 2) rationality; and finally, 3) individuality.

Independent existence

The human person is a substance. As a substance the person has existence in virtue of himself. This means that the person has independent existence – it is the kind of being which does not depend on any underlying being for its existence. This is best seen in comparison with the other categories of being, such as quantitative or qualitative being, both of which categories of being depend for their existence on the underlying being of a substance.

For example, a qualitative being like colour depends on some substance in which it can inhere, i.e. colour never ‘floats about’ by itself but is always found on the surface of some existing substance. In the very same way, all categories of being (excluding substance) depend for their being on the underlying existence of substances and are therefore secondary in character – Aquinas calls them accidents (because they happen in substances). Substances ‘sub-stand,’ stand under, all other kinds of being, thus providing for the possibility of their existence. The human person is such a being, a substantial being, not an accidental being.

Now, whereas the term substance highlights the underlying manner of personal, as the kind of being which underlies all other categories of being, Aquinas’ favoured term in the definition of the person is ‘subsistence.’ Subsistence focuses our attention not on the fact that the person underlies the other categories of being, like quantitative and qualitative being, but rather on the fact that the person is an independent kind of being.

Thus, in using the word subsistence instead of substance Aquinas puts the focus squarely on the kind of being of the person, on the fact that it "exists in and through itself and not in another." The person is a being that stands in being in virtue of itself without any anterior dependence (though obviously not disregarding the radical dependence of all being on God).

Rational Life

Yet, the person is not merely a substance, but is a living substance, and as living manifests rational life, and so life in the highest mode. First, as a being animated with life (including self-nutrition, growth and reproduction, sensation and movement thinking and willing), the person has a natural superiority over all non-living substances; then also, as a being manifesting life of the highest kind, rational life, the person has superiority over all non-rational creatures

Thus, the person has existence in the way that is most primary and most proper – he has being in the highest degree, and so is a perfection of being, a good in itself.


The human person is a rational substance. The essence of the human person, that which specifies the human person amongst the entire animal kingdom (of which he is a part), and that which sets him apart as a species, is to be found in his rational nature. Here rationality should not be understood according to a rationalistic or scientific caricature of reason: as something clinical, cold and calculating, something which could be adequately measured by an IQ test. Rather, rationality is created life at its apex—it is spiritual life.

How so? Well, according to Aquinas, and the Christian philosophical tradition before and after him, rationality is the ability of the soul to become all things. Here he closely follows the teaching of Aristotle and explains that the rational principle "has an operation extending to universal being." The rational principle empowers the person to read inwardly (intus legere – from which we get the word, intellect) the nature of a thing, and thus to "penetrate into the very essence of a thing." In the words of Aristotle, "everything is a possible object of thought," and "the soul is in a way all existing things." Everything can be thought, and the person, through the rational principle, can think all things. As such, it reveals that the human soul has a principle of activity which is not determined by material reality, for if it were so determined it could not know all material things.

The human soul

The human ability to know all of material reality, precisely as manifest in rational activity, reveals the immateriality of the human soul. But something that has immaterial existence is by definition spiritual. Immaterial – non-materiality – is simply a negative way of saying spiritual; spiritual is the positive designation of immaterial being.

In this way, the rational principle enables the human person to reach an intimate and deep knowledge of all existing things. In this very way, the person is capacitated for a fundamental and profound relationship with all of created reality. It is precisely this intimate communion with reality that we call truth.

Intellect and Reality

Aquinas defines truth as "adaequatio rei et intellectus,"the adequation of intellect and reality, the conformity of mind and things. Truth is the proper object of the mind – the object of thought is truth. Truth is had by the mind when its knowing is conformed to reality, when the mind, insofar as it is capable, knows reality as it is in itself. This occurs when the mind – in its judgements, concepts, and propositions – conceives adequately of reality.

Reality is the measure of the mind, and the mind achieves truth in coming to an ever clearer understanding of reality. In this accomplishment of truth, the mind is perfected. And so, for Aquinas, the human person has a necessary and fundamental relationship to truth – one could say that truth defines the human person.


In addition to this knowing power of the rational principle, a further constitutive dimension of rationality is the power we call the will—and with it, the property of freedom. Aquinas defines the will as the "rational appetite," or as we could otherwise translate: rational desire.12 The good is that which is desirable precisely because it is perfective of the one desiring.13 Simply put: all we perceive and know we can also desire – for all that is knowable is also in some respect good.

Rational desire is the greatest and highest of human desires for the good, since it can reach after the specifically human good: the true good, the only good commensurate to the dignity of the human person. In the consistent choice of the true good, the will is perfected. Thus, as with truth, the human person has a primary and foundational relationship to the good – the good defines the human person.

Importantly, this relationship of the person to the good is not determined, but is properly free. Freedom, as expressed in thought and creative action, distinguishes the human person amongst the creatures of created reality, and so too it makes the human person eminently individual.


The human person is uniquely individual. To be individual a being must be one and whole, indivisible in itself, yet divided from all else. All substances are individual in this way, yet rational substances are individual in an unparalleled fashion, precisely because they "have dominion over their actions."14 Aquinas says that persons are individual "in a more special and perfect way"15 because they are master of themselves through freedom of thought and freedom of action.

Unlike other creatures, which act only out of their nature, persons are the source of activity properly their own. Sub-personal creatures act according to a certain necessity of design and structuration, whereas persons act out of self and so are the origin of their own activity.

The will is the source of human action, action which is rationally determined and freely chosen. And it is precisely through the power of the will that the human person can act from out of his very self – out of his own freedom and power – and so can have action which is identified as his very own.

Taking Responsibility

As a result of this freedom, the person has the possibility of taking responsibility for himself. It is only because the human person is rational and free that can he take responsibility for himself, and so too, can be held responsible, both for himself and for his attitudes and actions. In traditional terminology, it is said that the human person is dominus sui: master of himself. It is not surprising, then, that for Aquinas, this mastery of self makes the human person an individual in a preeminent way, in a way surpassing all non-rational substances.Obviously then, this taking hold of oneself in responsibility is incredibly important for all of human life, both personally and socially. It is that which capacitates the human person for love, as well as that which conditions all human social, cultural and legal structures.


In focusing our attention more explicitly on this individuality, it is worth emphasising two separate yet related aspects: the person is a) incommunicable, and b) irreplaceable:

a) As a rational and free substance the human person is the bearer of his own existence and life, and his mastery of self simply cannot be assumed by another. The personal core – the interior freedom of the person as source of his own thought and action – cannot be taken, nor can it be given away. It is incommunicable.

b) A human person is always this very person, this concrete, historical individual. The person is not merely a multiple of a specific kind of being, but is always a properly unique existing individual. This means that the human person does not admit of replacement, he or she is irreplaceable.

These truths are intuitively available to all – and we grasp them precisely in the experience of personal love: When we love a person, it arises from the very centre of our being and freedom, and it is never love of just any human individual, someone who can be swapped-out, so to speak, and replaced with another. No, it is always MY love of THIS very person. Love is the most personal of actions: it affirms and delights in the individual as such, simply for being who he or she is.











Robert McNamara lectures at the International Theological Institue, Gaming, Austria

Faith Magazine

September-October 2016