In the 1930s an approach to Thomist metaphysics emerged which emphasized the importance of esse. Esse, understood as the act of being (actus essendi), was construed as existential act and as such was distinct from and responsible for the reality of essence. Essence, on the other hand, merely specified the material and/or formal characteristics which categorised a thing within its species and genera. Essence was complete in its own order but subordinate to esse because it was in potency to esse. Esse alone could render essence actual in the existential order and thus able to exercise its proper function.

The scholars associated with this approach, such as Étienne Gilson, Jacques Maritain, Joseph Owens C.Ss.R., and Armand Maurer C.S.B., became known as ‘Existential Thomists’. John Knasas, a student of Owens, is their worthy successor and in this book, Thomistic Existentialism and Cosmological Reasoning, employs an account of esse as prior to and an attribute of essence first to defend a version of the cosmological argument and second as an interpretive key for the rest of Aquinas's arguments for the existence of God.

The first part of the book contains five chapters. In the first chapter Knasas assesses Leibniz's version of the cosmological argument and Kant's criticism of it. Knasas agrees that Leibniz's argument is defective (pp.25-26), but instead of abandoning cosmological arguments per se prefers to seek a ‘more nuanced understanding of existence’ (p.26) which will avoid Kant's criticism. The second chapter addresses that task by examining the proof for the existence of God in chapter 4 of the De ente et essentia. It pays particular attention to the role of esse in that proof, suggesting it provides the ‘more philosophically nuanced view of the thing's existence’ (p.32) Knasas had identified as desirable. It emphasises the role of judgment, the second act of the intellect, in grasping esse intellectually. It suggests esse is both prior to (p.36) and an attribute of (p.60) the substance it renders actual and from there reasons to the existence of subsistent esse (pp.59-63), God (p.31). The next three chapters focus on a number of alternatives to Knasas's approach and argue for its superiority.

In the second part of the book Knasas uses his account of esse to interpret the rest of Aquinas's arguments for the existence of God. He begins in chapter six by arguing that natural philosophy cannot demonstrate the existence of God and that doing so belongs exclusively to metaphysics. In chapter seven Knasas broadens his approach. Not only can one use the priority of esse to reason from existing substances to the existence of God, but one can also focus on the type of nature esse is prior to and construct parallel arguments from the presence of form in matter (p.190), from accidents (pp.205-213), and even from motion (pp.213-215). The next three chapters apply Knasas's account to the proofs for the existence of God found in the Summa contra Gentiles (chapter 8), the Summa Theologiae and the Compendium Theologiae (chapter 9), and the other proofs Knasas finds throughout Aquinas's work (chapter 10). Chapter eleven addresses some further objections, whilst the final section provides the book's conclusion.

Knasas's book is undoubtedly the most impressive account of Existential Thomism available from a contemporary author. It positions Existential Thomism contextually within the views of a wide range of Thomist and non-Thomist philosophers. It also seeks to make the concerns of Existential Thomism relevant to contemporary philosophy of religion. For all its strengths though, it is likely the book's readership will be experienced Thomists and it is equally likely the main points of discussion will be ones already debated within Thomist metaphysics and natural philosophy.

For instance, Knasas frequently insists esse is prior to its subject, essence (e.g. pp.36, 37, 62, 68). If by this Knasas means a priority in the metaphysical order, then that priority will either be a priority of time or of nature. Neither option appears viable, though. A temporal priority is straightforwardly incompatible with Aquinas's claim in De Potentia Dei that ‘Deus simul dans esse, producit id quod esse recipit”’(Q3 art. 1 ad 17), at least if one reads ‘id quod esse recipit’ as essence or subject. If God simultaneously produces esse and essence, then esse cannot be temporally prior to essence or subject.

On the other hand, if Knasas opts for a priority in nature, how does one account for that group of texts which suggest form is prior to esse (e.g. ScG II c55, ST 1a Q.50 Art.5, ST 1a Q.75 Art. 6)? If esse follows form, then form will be prior in nature to esse. But since form also pertains to essence, if form is prior in nature to esse, then so essence will be. Knasas glosses such texts by suggesting form is a condition of esse (pp.62, 101) and the complementum of substance (pp.55, 113). However, neither gloss is convincing. Conditions can be prior in nature to that whose conditions they are and the priority of form over esse is not threatened by form completing substance, it just means essence will also be prior to esse as we have already argued. Aquinas links form as principle of esse with formal causality, thus confirming form as prior in nature to esse, whilst insisting God's efficiently causal role vis-à-vis esse is prior to form: ‘esse naturale per creationem Deus facit in nobis nulla causa agente mediante, sed tamen mediante aliqua causa formali: forma enim naturalis principium est esse naturalis”’(DV Q27 art 1 ad 3). In the metaphysical order, Aquinas's order of priority is God, form, esse. Any priority esse enjoys at best pertains to the creator's order of intention.

Another concern relates to considering esse as an attribute. If we do, will we not end up reifying esse? Knasas insists that esse is a sui generis attribute (p.139) but suppose one imagines Socrates alive in Athens in 399 B.C. This individual, Socrates, is an essence rendered actual by his esse: Socrates is an actualised essence. Yet if one treats esse as an attribute, then the actualised essence which Socrates is will just be an essence together with an attribute, its esse. This pushes the criterion for the real distinction of esse and essence towards potential separability. An attribute and its subject are separable even if in the unique case of esse the subject would not survive such a separation. It also weakens the unity of Socrates because if one had encountered Socrates so construed one would just encounter Socrates’ actuality conjoined to his essence. That does not seem to capture the full reality of encountering Socrates or anyone else for that matter. Nor does it explain the limitation of Socrates's esse. If Socrates's esse is just attached to his essence, as opposed to being received in that essence, how does that attachment limit esse?

One might also question Knasas's claim that motion is an accident with its own esse (p.214). It is one thing to say accidents have esse, quite another to say motion is an accident and has esse. There are far too many differences between accidents and motions to justify construing motion as an accident. Motions are imperfect acts, accidents are perfect (complete) acts. Motions are the progressive actualisations of potencies for becoming. Accidents are the completed actualisations of potencies for being. The reddening of an apple is not the same as the red of an apple. Growing to 6’ is not the same as being 6’. Travelling to London is not the same as being in London. In order for motion to be an accident it would have to be stable in being. It is not, though; therefore, it cannot be an accident and it certainly cannot possess esse.