The purpose of this section of the dissertation is to assess the minimum level of certitude for the thesis proposed, with an additional comment of any suspected higher level of certitude. There are various levels of certitude that can be chosen. Opinion is defined as intellectual assent (or disagreement) given to one part of a contradiction with fear of the opposite.⁵¹ Possibility is defined as the capacity for existence for a concrete possible thing: internally, that its constituent characteristics are not impossible, and additionally externally possible, if there is power to produce the thing.⁵² Probability, also called likelihood, is defined as the weight of motives, or the accumulation of serious motives, for prudent assent to some proposition, which is intrinsic probability if the motive arises from the nature of the thing, and can be extrinsic probability if the motive is from authority, which can also suppose the internal motive.⁵³ Summary of Probabilities is defined as an accumulation of probable arguments, considered according to their force, which results from a mere juxtaposition. Convergence of Probabilities is defined as an accumulation of probabilities which converge to produce a sufficient reason. Moral certitude is defined as firm assent to one part of a contradiction whose necessity arises from the moral law in the physical (not ethical) sense, e.g., every mother instinctively loves. Physical certitude is defined as firm assent to one part of a contradiction whose necessity arises from the very physical nature of the thing, e.g., the law of gravity. Metaphysical certitude is defined as firm assent to one part of a contradiction whose necessity arises from metaphysical necessity, e.g., my own existence.⁵⁴

Certitude could arise from some observable fact or experiment. Marcozzi, at the Gregorian University in Rome, notes that the a posteri method is founded on the essentially evident principle that every being cannot manifest something it does not have; so there is a possibility of knowing the operator, at least in part, from its operations.⁵⁵ Experiments⁵⁶ show that anthropoids can go through an obstacle course, remove impediments to get food, use diverse objects to attain a goal, and in some cases animals can adjust the means for the goal; however, none of these attainments involved the use of reason for systematic research, from hypothesis to experience to the correct solution. Animals were either trained, or saw the solution previously, or used instinct, or succeeded by chance attempts. "Observation and experiment show that animals even the most elevated have manifestations, even marvelous, only materially; men instead, purely spiritual," notes Marcozzi.⁵⁷ Marcozzi notes the fact that all men have a superior culture, is obvious and needs no demonstration.⁵⁸ This is confirmed by Arcidiacono, at the Gregorian University in Rome, who states that "Mathematics produces technical achievements that liberate man from the servitude of material."⁵⁹ Babolin, from the same Gregorian University, notes the unique achievements of man in art and culture, education, morality, and religion.⁶⁰

Certitude could arise from some philosophical explanation that exists. Explanations were given by several Neo-Scholastics. Nogar notes that man is unique and is essentially different from the animals.⁶¹ Calcagno illustrates the physical discontinuity between man and the apes.⁶² De Finance notes that animals have only biological goals, but man has a horizon, an ultimate term of reference into which he can insert his perceptions into a wider totality.⁶³ Di Napoli notes the disjunction between men and apes is both physiological and psychological.⁶⁴ Donat argues that the human soul is essentially diverse, and so is higher in every order from the beast.⁶⁵ Donat also argues that animals lack intellect, since there is no intellectual activity: no tools, no art, no education, no progress, no cognition of supra-sensibiles (e.g., honesty, obligation, religion), and no ability to match means with the proper goals.⁶⁶ Gredt argues that brutes do not have an intellect, but only a sensitive soul (anima sensitiva); and Gredt also argues that brutes are unable to connect cause and effect.⁶⁷ Hoenen argues that since all the intrinsic operations of the animal depend on matter, consequently also does the substantial form of brutes.⁶⁸ Iturroz argues that animals do not formally know the nature of the goal and the means, nor in the goal do the know the useful relation between the means and the goal, nor do animals discern an object as convenient in itself or because of another.⁶⁹ Klubertanz argues: "The object of the sensory appetite is a good known by sense; the object of the will is the good known by the intellect; hence the power of appetite that is proper to man is in the will" essentially different from animals.⁷⁰ La Vecchia notes that, "Differently from the lower animals, man not only knows, but man knows that he knows...He is then essentially more elevated and more perfect than any other organism."⁷¹ Marcozzi argues that the ideas of man are intrinsically independent of material.⁷² Palmes argues that animals lack an intellectual life by which the sense life is completed and man is raised to a higher prefection; Palmes also argues that animals do not have the true free will.⁷³

Certitude could arise if the argumentation was based on some philosophical principles.

The principle of causality would be violated if animals where able to function on an intellectual level as men do; because the effect (language, progress, conceptual thought) would be greater than the cause (anima sensitva).⁷⁴ The principle of sufficient reason would be violated in animals for the same reason, but would be satisfied in mankind because the intellectual soul would be the sufficient reason for human progress, language, art, culture, morality and religious practice.⁷⁵

Certitude could arise if the explanation is sufficient, due to the principle of sufficient reason. But the explanation is based on the vital principle of man and the vital principle of brutes.⁷⁶ The spiritual vital principle of man is not able to be reduced to the material vital principle of brutes. Likewise, the material vital principle of brutes is not an adequate cause for the spiritual effects seen in man alone. Therefore, the explanation of the essential difference between man and brute satisfies the principle of sufficient reason.

Certitude could arise if the explanation was rooted in St. Thomas Aquinas, thereby being faithful to tradition. St. Thomas maintains an essential distinction between man and animals, because animals only know the singular by way of sense, while man can know the universal by intellect. St. Thomas notes: "Only rational created nature has an immediate relation to God: because other creatures do not attain to the universal, but only to the particular, either by participating in divine goodness by just existing, just as inanimate things, or else by living and knowing singulars, just as plants and animals. Rational nature, as far as it knows the universal nature of the good and of being, has an immediate order to the universal principle of being" (Aquinas Summa Theologiae 2-2. 11. ad 3).⁷⁷

Certitude could arise if Neo-Scholastics agree on the fact of an essential difference between man and the other animals. Norgar, for example, notes "Man is different in kind, not just in degree."⁷⁸ Other Neo-Scholastics who agree have been mentioned with the arguments they proposed for the essential difference between man and brute. These Neo-Scholastics are: Calcagno, De Finance, Donat, Di Napoli, Gredt, Hoenen, Iturrioz, La Vecchia, Macozzi, and Palmes.

Certitude could arise due to recent scientific confirmation by convergent scientific arguments. Regarding the term "intelligent," some authors, who consider the sensitive life of animals, describe the perfection of animal activity as more or less "intelligent."⁷⁹ This way of speaking is entirely inappropriate, arises from false doctrine, errors and confusion, and is without philosophic proof; such use of "intelligent" ought to be avoided in writings on experimental science. Nogar notes that there is a distinction between the psychosocial factors of man and his biological, or genetic, factors, so that the psychosocial is not reducible to the biological.⁸⁰ Köhler comments on anthropoid experiments, which illustrate the exclusively material operations of animals, that the object has to fall in the same visual field, so that the necessary means have to be in the same visual material field as the goal.⁸¹ Révész, who raised a baby chimpanzee with his own baby son, noted that even with an enormous number of experiments, he was never able to find even the minimum traces of the comprehension of a problem in animals.⁸² Marcozzi notes that the scientific study of fossils and living Anthropoids today has shown no change in 10 to 15 million years (from the Miocene), with no utensils, no weapons, no huts, no spiritual life or conscience.⁸³

Certitude could arise if the opposite opinion is not tenable. The opposite opinion is not only not tenable (as proved from metaphysical and also from a posteri arguments), but the opposite opinion is also dangerous. No one can be indifferent to the allegation of a mere qualitative distinction between man and the animals, instead of an essential difference between man and the other animals. The lack of an essential difference between man and other animals would not only degrade the honor and the good name of our own race, but there would be real danger of minimizing human dignity.⁸⁴ So the question of complete and total solidarity with other animals is not a speculative one, but one connected with the nature of man. If man is only and merely an animal in nature, then human dignity (and with it morality and culture) is indeed at risk.

Certitude could arise if the objections of adversaries are able to be answered. However, the objections of the adversaries can be answered.

OBJECTION: Your arguments prove that animals do not have human intelligence, but they do not lack all intelligence.⁸⁵ For example, animal intelligence is real but less perfect; second, even men have distinct grades of intelligence; third, newborn humans have less intelligence than animals of the same age; fourth, men have less intelligence than angels, but still have real intelligence. REPLY: The assertion is denied. First, animal cognition is only of the sensitive order, and not just "less perfect." Second, although there are grades of human intelligence, all men have abstraction for universals, reasoning, language, progress. Third, the argument concerns definitive intelligence, not the growth of intelligence in infants. Fourth, although man’s intelligence is less than the angels, animals do not even have the lowest level of intelligence, but only sense cognition and instinct.

OBJECTION: In particular, the thesis does not prove from defect of progress, since animals do learn, for example, sheepdogs.⁸⁶ Further, if animals do not have progress, this is not a lack of intellect, but perfection of intellect that sees progress already attained. REPLY: The assertion is denied. Even if animals learn, they do so by repetition, not by intellectual explanation. Further, even if the progress of animals is most perfect in the sensitive order (by instinct), this progress cannot be applied to higher goals (rationally chosen by men).

OBJECTION: The thesis is not demonstrated by a lack of ability for conceptual language, for animals have natural language.⁸⁷ Further, the natural language of animals is truly conceptual, for it is described in comparison to human language. REPLY: The assertion is denied. Their natural sounds prove the animals have what is needed for conceptual language, but there is no conceptual language. Further, if the natural language of animals was truly conceptual, it would be the animal that communicates the concept, instead of the human researcher describing it.

Certitude can be had from the possibility of philosophers and theologians admitting this mode of origin without damage to their other beliefs. Modern philosophy, even the most recent, accurately tends to make a distinction between the human person and all the rest of corporal beings. Therefore, man is called a subject, and the rest of corporal being are objects; man is a person, and the rest are things. Accordingly, no philosopher should be disturbed by the those who hold an essential difference between man and brutes, or even man and the rest of the world.⁸⁸ No theologian would be disturbed by the essential difference between man and brute, since the theologian already endorses the superiority of mankind, due to the direct and immediate creation of the human soul by God.⁸⁹

The level of certitude for "Man is essentially different from the other animals" is at minimum at the level of the metaphysically certain. The proof is the principle of causality. Further, the convergence of all of the above arguments are proof, especially the fulfillment of the principle of sufficient reason. This agrees with the opinion of Hugon who holds the thesis as "certain."⁹⁰ This also agrees with Klubertanz, who notes in the analysis of animal activity, "Everything that is necessarily implied by a given activity, can be asserted to be present with certitude."⁹¹

Having come to the correct conclusion on the philosophical level of certitude, the philosopher must still conclude with some humility. The philosophy of nature does not disregard the objects observed and perceived by sense.⁹² This is the method of Aristotle and St. Thomas.⁹³ Man is an animal⁹⁴ and shares the sensitive operations of animals, so that our knowledge must come from similarity or dissimilarity.⁹⁵ Further, we only know the nature of man and the other animals by their activity, because direct knowledge of anything is hidden.⁹⁶ So it necessary to know things indirectly by a knowledge of their activity or manifestations, avoiding the error of those who only seek facts and not conclusions, or the error of those who, without first consulting nature, construct their system a priori.