The fundamental question that drives the whole complex system of the Critique of Pure Reason is the following: Is it possible to formulate synthetic judgements a priori?
Our journey through the history of philosophy so far has repeatedly witnessed changes of course that have not only radically altered the entire trajectory of philosophy, but in some cases completely overturned it. Without a doubt this is true for the work of Immanuel Kant (1724-1804), to the extent that he himself calls his own philosophy a ‘Copernican revolution’ – and in this case there is no evidence of arrogance or exaggeration.
Some months ago, we left off with the Humean critique of the principle of cause and effect.  Kant recognised Hume as a mentor, and in the Preface of one of his important works he wrote: «I freely admit that the remembrance of David Hume was the very thing that many years ago first interrupted my dogmatic slumber and gave a completely different direction to my researches in the field of speculative philosophy».  In order to fully understand this turning point, it is necessary to give a theoretical framework of the state of the art during Kant’s time. The standard reference book for metaphysical philosophers in Germany and elsewhere was Christian Wolff’s German Metaphysics [Deutsche Metaphysik 1720], which contained various essays, almost all on mathematical subjects, plus a text on logic. Wolff, an important exponent of the German Enlightenment, understands metaphysics as a science that concerns everything that is possible, i.e., everything that is rationally conceivable; in this respect, every factual truth (or factual knowledge) must be traced back to a truth of reason. Therefore, it is first necessary to establish a solid logical doctrine based on the law of non contradiction and syllogism as a deductive method. As for the structure of ‘metaphysical science’, Wolff sees ontology as preliminary to the so-called special metaphysics, which is divided into three sections dealing with three specific ‘entia’: the Soul, the World and God. What Kant does not approve about this kind of metaphysics is its scientific claim while lacking a solid foundation established by deep consideration of gnoseology and the very possibility of scientific knowledge based on empirical evidence. But let us delve into this theme by looking directly at Kant’s work.
Kant’s problem stems from the realisation that metaphysics poses itself as a science, while at the same time it seems to be subject to the personal arbitrariness of every philosopher who approaches it – be he a true expert or a mere bluffer. Many have been the various metaphysical systems, and so many have been contrasting ones that one wonders whether they really concern the same issue, given how it seems impossible that so many minds could disagree on the same subject. The fundamental questions to be asked are therefore the following: Is metaphysics generally possible? And is it possible for it to be established as a real science? Indeed:
A ‘dogmatic’ approach to metaphysics is no longer possible: it is necessary to start from the cornerstone, like Descartes had already tried to do in order to understand first and foremost, how something can be ‘known’ in general, and subsequentially whether such a gnoseology allows the establishment of any metaphysics that can be rightly and indisputably defined as ‘science’, i.e., capable of giving undeniable and universal answers. What would be interesting here is a comparison study on the first edition of the Critique of Pure Reason [Kritik der reinen Vernunft 1781], which uses a synthetic procedure, with Prolegomena to Any Future Metaphysics That Will Be Able to Come Forward as Science [Prolegomena zu einer jeden künftigen Metaphysik, die als Wissenschaft wird auftreten koennen 1783], written for those who had found the Critique too unclear and incomprehensible, and compare them in tandem with the final version of the Critique of Pure Reason, published in 1787. Unfortunately, this is clearly impossible, hence we shall proceed very briefly, presenting only the fundamental ideas from which Kant’s work takes its cue.
Let us begin by saying that for Kant knowledge is based on two types of judgements: analytical and synthetic. The verb ‘to judge’ for Kant indicates the activity of knowing, because ‘judgement’ is the link that binds the subject of a proposition to its predicate; this terminology derives from the logic of the Port-Royal school, which focused not so much on the linguistic aspect of a proposition, but on the connections between signifieds. Therefore, the connection that can exist between a subject and its predicate is of the analytic type when the predicate merely manifests something that is already present in the subject from the start (e.g. “a ‘triangle’ has three angles” is an analytic judgement because the term “tri-angle” already contains the predicate “having three angles”); whereas it is synthetic when the predicate is not contained in the subject, and thus a new connection is established between two previously unrelated signifieds (e.g. “the triangle is red” is synthetic because the picture of a triangle does not contain the predicate “being red” in itself). It is evident that from the gnoseological point of view the two judgements have a very different nature: the analytical judgement is fundamentally a priori, i.e., it goes beyond the empirical knowledge of the object judged, and for this reason it is always and universally valid. However, it also has the defect of not really expanding our knowledge of the object: all it does is explicate what we already knew implicitly. Synthetic judgement, on the other hand, is fundamentally a posteriori, i.e., in order to be constituted and verified it needs empirical knowledge of the object and given the incidental nature of the occurrence of entities to our senses, it is never in itself universally sound; nevertheless, its merit is that it creates a link between subject and predicate that could not be established a priori, and hence is the privileged method for expanding knowledge. In order to constitute a solid and indisputable gnoseology, that is to say, in order to establish any metaphysical science, there would need to be a judgement that is capable of fusing the merits of both together, namely, give universally valid answers and at the same time enable new connections to be established: an a priori synthetic judgement would be needed.
Therefore, the fundamental question that drives the whole complex system of the Critique of Pure Reason is the following: Is it possible to formulate synthetic judgements a priori? If the answer is affirmative, then it will be possible to establish a metaphysical science that will be no less thorough or valuable than the so-called natural sciences. However, to answer this question, it is first necessary to demonstrate that a rationality whose mechanisms are constituted regardless of empirical experience exists; and second, it is necessary to fully grasp its nature and method. Indeed, the term ‘critique’ used by Kant here means ‘study of the conditions of possibility’, while ‘deduction’ stands for ‘rational justification’. This need to analyse ‘pure reason’ arises in Kant from the idea that synthetic judgements a priori do exist and are for instance mathematical ones. In rationalistic tradition, the sum 5+5 already contains within itself the result 10, and thus it is an analytical judgement; for Kant, instead, 5+5 does not contain 10 within it, but rather that the result is fruit of experience acquired by concretely adding two groups of objects of five elements each and noticing that the result is always a group of ten elements. Therefore, this judgement is synthetic (because it expands knowledge) and remains universally valid: for Kant it is a scientific judgement. The keystone is that the universal and necessary connection of subject and predicate does not come directly from the experienced object but rather is the work of the experiencing subject. In other words, the faculty of unifying those multiple and confused data that sensibility perceives into a universal synthesis belongs to the knowledged subject: it is a faculty that pertains to subjectivity. Furthermore, it is only by analysing this faculty of subjectivity that one can understand how to construct a science with necessary and universal answers. Consequently, the entire Critique of Pure Reason analyses the faculty of knowledge of the ‘I think’ (subjectivity), and this is the Copernican revolution operated by Kant that will be crucial for subsequent philosophy. But a word of caution, however, do not understand this subjectivity in a psychologistic sense, or one dominated by arbitrary and individual influences: subjectivity is not individuality, it is the structure of knowledge peculiar to human rationality as such.
I shall now give a brief overview of the structure of the Critique of Pure Reason, which unfortunately will only be outlined here. First, let me specify that for Kant the thing in itself (the substance of the entia present in the world) is unknowable and not perceivable through our senses. In fact, Kant calls entia, ‘noumena’, and they are conceivable but not knowable; conversely, what is perceived with our senses are the ‘phenomena’, i.e., the objects of representation that are cognisable through experience. To put it less elegantly, we could say that a noumenon is the object that cannot be known, while a phenomenon is the representation of that object that is both cognisable and perceivable through our senses. This knowledge occurs thanks to a complex gnoseological structure, which starts from the a priori forms of Sensibility: Space and Time. Every object can be known by the subject thanks to pure intuition, i.e., its being located within time and space; these issues are dealt with in Transcendental Aesthetics. Sensibility provides a series of data that must, however, be systematised in a coherent and rational order. This work is carried out by the Intellect, which reorders the data under a common representation, the concept. The concepts of the Intellect – also known as categories – are not acquired through experience but are innate in subjectivity and thus the Intellect operates spontaneously. The categories can be identified by analysing the different forms of the propositions that are used to express judgements, there are twelve of them and they are divided into four main groups: Quantity, Quality, Relation, Modality. The great problem is to explain how it is possible that the Intellect possesses these concepts a priori, i.e., regardless of our sensibility, and that at the same time they are so effective in processing sensible data. This is the precise point at which the Kantian ‘I think’, basically isolated from the world, must justify the legitimacy of the application of these categories to sensible data, i.e., show that they are adequate to the task and not inadequate, and therefore misleading. What is needed is a ‘transcendental deduction of categories’, which Kant attempts, but which will always be the most problematic point of his entire system; these issues are dealt with in Transcendental Analytics. Finally, if the ‘I think’ merely grasps sensible data and processes them through innate tools, how is it possible to make judgmental mistakes? The fault lies in Reason, which tends to apply the categories of the Intellect to an unlimited number of phenomena that cannot be effectively experienced by the limited capacities of the ‘I think’. One could say that Reason’s sin is its lack of measure: it would like to make judgements about phenomena that cannot be experienced, or at least not to an adequate degree. The consequence of this way of operating is the appearance of ideas that are absolutely unjustifiable because they cannot be rationally justified in terms of their existence, their nature, and their attributes; these issues are dealt with in Transcendental Dialectics.
In conclusion, it is worth noting that the three most important ideas of Reason, i.e., the most illusory and scientifically inconsistent, are the idea of the soul, the world and God. Kant excludes without appeal from the sphere of possible knowledge and thus, ipso facto from the sphere of competence of metaphysics as a science the so-called ‘antinomies of Reason’. These are exactly the basic concepts of that rational psychology, rational cosmology, and rational theology that constituted the three sections of Wolff’s special metaphysics– and metaphysics as it was interpreted by Kant’s contemporaries. This simple detail should suffice to demonstrate how original and innovative the thought of the philosopher from Königsberg was, revolutionary to say the least.
 Immanuel Kant, Prolegomena to Any Future Metaphysics That Will Be Able to Come Forward as Science, with Selections from the Critique of Pure Reason, Translated and Edited by Gary Hatfield, University of Pennsylvania, Revised Edition, Cambridge University Press, New York 2004, p. 5.
 Ivi, p. 10.