The first original formulation of Abbagnano's thought came after his early attempts to oppose Croce's and Gentile's idealism, by appealing to the "irrational sources of thought" and to a revaluation of the philosophical importance of scientific knowledge. This formulation was tightly bound to the introduction of the philosophy of existence to Italian philosophic culture, that was taking shape in the 30's, above all in Turin and Milan, and further to the debate about it that went on before and after the war. In 1939 Abbagnano published La struttura dell'esistenza a book that, as Norberto Bobbio wrote thirty years later, "bore no resemblance to any other philosophical works written in those years, not even in its form which was concise, linear, without the usual oratorical excess and the usual dialectical virtuousness". It was a book, which "was not easy", but " just because it had been written with exactness, guided and supported by a rare intellectual discipline, it was easily understood". Like for Heidegger and for Jaspers, for Abbagnano too, philosophy is a search for being, and this search is began by questioning man, who is defined in his existence by the possibility of stating the problem of being, that is, by the struggle toward being". But Abbagnano clearly distinguished his own philosophical position from the two great German representatives of the philosophy of existence and, above all, he kept away from the negative results to which, he thought thy would come. According to Abbagnano, Heidegger considers the effort towards being "from its starting point" that is according to its original situation such that "existence appears to be an existence from nothingness". "Nothing" therefore becomes "the term that defines existence and establishes it in its true nature". Jaspers instead considers the struggle towards being "with respect to its final situation" conceiving being "beyond the effort the move towards it" that is, as transcendence and thereby unattainable by a search. The conclusion is that the search for being becomes, for Heidegger, "to exist for nothing (ness)" and, for Jaspers "the realization of one's own impossibility". This means that, in the first case, it is marked by " dread" and by "failure" in the second case.. Thus Abbagnano rejected both Heidegger's identification of true existence with "living for death", and Jasper's conception of being as unattainable by man such that philosophy aims at being without the chance to reach it and, still less, to give it form. He opposed Heidegger's and Jasper's existentialism by stating that "the final situation of the effort towards being, achieves its essential unity with the initial situation". To characterize this unity he appealed to the notion of structure as it had been enunciated by Dilthey. Existence has a structure of its own, but this is not given to man; it appears instead as a "possibility to be achieved". In fact existence is a possibility and is based on the possibility of possibility, that is, on a transcendental possibility.

In opposition to Heidegger and Jaspers, but later also to Sartre in L'être et le néant, Abbagnano suggested a positive existentialism, aimed at underlining the problematic character of existence and human commitment. In this regard he places great importance on the notion of freedom and its connections with the notion of choice. According to Abbagnano, man is defined by the possibility to choose among the possibilities he is offered to him, to achieve some and not others. That is the meaning of transcendental possibility. Man can choose between authentic or inauthentic existence, between fidelity to his own being and the dispersal of every day life. The possibility of choice, that Abbagnano says is given to man thus acquires the character of a norm - a norm which Heidegger had rejected: "The norm is the 'dover essere' of freedom as transcendence", a duty that coincides with the duty of achieving one's own structure.

Against idealism, e. e. both against the dissolution of the individual man in the process of achieving the infinite spirit (Croce) and dissolution in the pure act of thought (Gentile), Abbagnano claimed the finitude of man, his temporality, his freedom to choose among certain situations as well as the moral value of this choice. In the work published after La struttura dell'esistenza, that is, in Introduzione all'esistenzialismo and in the essays of Filosofia religione scienza and Esistenzialismo positivo, he recognized the fundamental problematic character not only of man's existence, but also of every reality. Philosophy itself is, for him, "a problematic knowledge or reason". This limited reason, aware of human finitude and of its relation with a world that is also problematic, is thus opposed to hegelian "justifying" reason. On this basis, Abbagnano defined the relation of philosophy on one hand, to religion and on the other, to science. Philosophy shows human beings the way of inquiry; religion the way of faith. As for its relation to science, philosophy is not knowledge of facts; it cannot therefore claim to offer a metaphysical integration of the results of science. Science covers the sphere of what can be known; philosophy does not have an approach to the world different from scientific knowledge. It is rather a "task" posed to man in virtue of his own existence.

This definition of philosophy and of its relation with science became the starting point of that phase of Abbagnano's thought, which he announced when he participated in the debates at the Centro di studi metodologici, together with Norberto Bobbio, Ludovico Geymonat and also with several mathematicians, physicists, and engineers. In 1934 Abbagnano had already dedicated a book he entitled La fisica nuova, which examined the theory of relativity, quantum theory and Heisenberg's principle of indeterminacy and tried to exolain their philosophical meaning. After 1945, he moved away from the themes of existentialism and concentrated once again on his interest in science. Against the pragmatic or conventionalist interpretations of science, Abbagnano asserted its cognitive value indeed its character as valid knowledge. Only science allows "to set in order" and then to know the world. Philosophy is not a form of knowledge different from science or an alternative to it, but it is rather a meditation on it. It is a gnoseological or epistemological enquiry. At the same time Abbagnano suggested an interpretation of scientific knowledge that would detach it from the determinism of the science of the XIX century, and particularly, from the category of necessity that classical positivism had in common with romantic metaphysics. On this issue he would return to the notion of possibility, that he had fomulated in his existentialist period. If XIX century science pursued the ideal of a causal connection of facts, that expressed itself in a system of necessary laws, then contemporary science instead set out in search of conditioning relations. This is true not only for physics, as regards the theory of relativity and the principle of indeterminacy, but also, and above all, for the social sciences. Whereas idealistic culture had ostracized sociology viewing it a "false scince", Abbagnano considered it an important contribution, that acknowledged social life as a fundamental dimension of human existence. It is essential for the individual human being to have a relationship with other individual human beings, establishing therefore, the possibility of communication. Sociology has the task of showing the concrete dimensions of this possibility starting from the behavioral framework that Abbagnano had identified in individual behavior.

The period of "New Enlightenment" in Abbagnano's thought began in 1948 with the article Verso un nuovo illuminismo. In Dewey's pragmatism and in neopositivism he found an inspiration towards a positive idealism. It consisted in considering reason "as a human force endeavoring to make the world more human". As a consequence, the program of a rational transformation of reality in the name of a "limited, but not impotent reason, coincided with a "methodological" conception of philosophy. This period ended in 1961 with the publication of Dizionario di filosofia. Since then Abbagnano was more and more concerned for the problems of daily existence and for a conception of philosophy as "wisdom", Platonic provenance. (In fact Platone was his favorite author among the ancient philosophers). After the failure of the New Enlightenment, in a cultural climate that had seen the success of a Marxism which took up the legacy of idealism, Abbagnano's thought was directed to common people, using an ever popular style. If "Philosophy is no comfort", according to the title on an essay of a student who had died young, it might at least help in the choices that everyone must make in everyday life, by showing the alternative possibilities, a task not directly related to the sciences, but to which scientific knowledge makes an indirect contribution, by means of the understanding of the world and of the human condition a contribution which is not inessential.