In the next step of the development when we come to Leibniz, a remarkable change takes place. We must remember that Leibniz not only inherits Platonism and Cartesian philosophy but he also inherits Christian philosophy and this brings a new element into the whole perception and this perception is very important and for the sake of understanding rationality, this importance should be underlined. It is the following.
In Christian theology there is a distinction made between two states of becoming and that is a very important distinction. There is according to Christianity the world as you see understand it and there is a heaven and both are process of becoming and yet there is a divergence between that and this. Now this perception of Christianity was not sufficiently amalgamated into rationality of the West, until we come to Leibniz. You will see that in the Western thought this distinction which is made by Christianity was in due course of time rejected and even after Leibniz who undertook to assimilate the Christian idea of the two kinds of becoming, this attempt which was made by Leibniz even that came to be rejected in the subsequent development of rationality in the West and therefore whenever there is a discussion about rationality, there is an attempt to understand becoming as we find it today but not in contrast to heaven. In the case of Leibniz, he clearly recognised the distinction between the two kinds of becomings. The becoming as we see now and the becoming that is in the heaven. He does not use these two terms, which I will come to shortly but I just wanted to underline this distinction because it is of fundamental effect for an ultimate solution of the problem, of rationality. One of the reasons why rationality has some kind of arrestation today it is because the distinction that has got to be made between various levels of becoming has been blurred, that is to say our understanding of the world is not blind but one–eyed. It is only when you open both the eyes that you see both the worlds or many planes of the world and our difficulty of rationality is that it stands to be a kind of an attempt to understand the world of becoming as we normally see and we don’t suspect even that there is something else, or if there is something else we try to feel uncomfortable about it. But in Leibniz, and this is what I would like to point out, that logically because Leibniz has been regarded as the greatest synthesiser in the history of Western philosophy. In fact Bertrand Russell regards him to be the greatest intellect in the history of mankind. And as we all know Russell himself wrote one full book only on Leibniz and that is because of the importance of intellectual history as represented by Leibniz.
Now what was his fundamental contribution to rationality? He distinguished between two logics. He said that Aristotelian logic was only a logic of the Law of Contradiction, Law of Identity and Extruded Middle. In other words it was only the logic of identity basically and therefore that logic cannot explain very well the phenomena of the becoming, that logic cannot easily apply to the world of becoming, it can apply to the understanding of the permanent, of the universal but when you want to understand the phenomena and the becoming, we cannot give a full account, a true account of it. Now, he spoke of these two logics as the logic of necessity and the logic of sufficient reason and this is the special contribution that Leibniz made in the history of thought. According to him the laws of thought which were enunciated by Asel are all laws of necessity. In other words rationality consisted of showing that parenthesis and conclusions, whether inductive or deductive the relationship should be necessary, that is to say: if you start from a universal proposition, – ‘All men are mortal’, ‘Socrates is a man’, ‘Socrates is mortal’; we arrive from one proposition to the other and the context of it is necessarily true. If you start from a valid proposition, conclusion must be necessarily true. Why necessarily true because it follows the Law of identity, it follows the law of non–contradiction, it follows the Law of extruded middle. Therefore the conclusion is necessarily true. In the inductive logic also, if you start with the particulars and you arrive at the necessary conclusion which is universal the conclusion is a necessary consequence. And till the time of Leibniz science claimed that if knowledge whatever is knowledge, was based upon inductive truth and inductive process, you started with particulars arrived at universals and all the universal laws gave you the knowledge of necessary consequence. Because of the Law of gravitation an object must fall necessarily from top to bottom, because of that the laws of tides of the ocean can be explained necessarily, the movement of planets can be explained necessarily. In other words at that time it was regarded that the scientific knowledge is also a knowledge of necessity, it explains all the necessary phenomena in the world.
Leibniz pointed out however that the world of becoming is not merely the world of necessity. There is also what we call the world of contingence. This is a discovery of Leibniz and he emphasised it that this world is not merely the world of necessity, there is something like contingence, it may happen, may not happen. According to the law of necessity, it must rain today evening. If necessity is the ruling principle everywhere, you may not understand why rain must fall today. But if you knew all the facts of the world, if you know the universal, if you know the permanent, if you know the connection of the permanent with the becoming then you can be sure that it must rain today. According to Leibniz this is an assumption, not true. Bertrand Russell while commenting on Leibniz’s theory says: There was no necessity that Napoleon Bonaparte should have been born on 15th of August 1759, there was no necessity. He was born on 15th of August 1759, why? There was no necessity, he was born, he happened to be born. So he said there is an element of contingence in this world. Now this is a proposition which can be challenged, which can be discussed, debated and in the Western field of rationality this is being debated. But it is a very important underlining of the fact that the world does not consist merely of necessary events, it consists largely of contingencies. But if contingency, the question was, does contingency imply chance? Has it happened by chance? Because contingencies happened, he was by chance he was born on that day. If it is chance then rationality goes off the board. There is no reason what so ever why he happens to be born on that day, no necessity. But even contingency, is it by chance? And his answer was, it is not by chance either altogether, neither by necessity, nor by chance. And he therefore gave a new dimension to understand the world rationally. So he said when you cannot explain anything in terms of necessity, you should be able to explain a thing in terms of what he called sufficient reason. So he introduced in the logic, in rationality the presence of what he called sufficiency of reason, sufficient reason. And what is sufficient reason? And he explained: an event is contingent but rational if first of all it must be a possibility. If the world is chance then even impossible things would happen but that is not the case according to him. The world is a world of contingency but that contingency consists only of an event happening because it is possible. So first of all an event must be possible. Rain must be a possibility, if it is not possibility at all, it will not happen. And secondly there must be some special impulsion for it. And this is an important element in the rationality that he introduced in the Western thought. It is very important. And what is that? According to him this world cannot be explained if there is not somebody who is controlling the events happening in the world, it doesn’t happen by chance. First of all there must be a possibility of an event to happen but even if it possible it must not necessarily happen. Then why does it happen what is the sufficient reason? So he says there must be a controlling agency through whom, through whose will an event must occur and if it is contingent that will must be such that it can chose one or the other to happen. And it must be such that it must be omnipotent, so that nobody else disturbs it. Therefore an omnipotent God capable of controlling many possibilities and freely deciding, this is the important idea, freely deciding an event to happen therefore that happens. So an event in the becoming in the whole world if you see, an event occurs because it is possible and secondly because it expresses the free determination of God, who allows that possibility to come into being safely. He went one step farther and he pointed out that this possibility is never alone in the world. God is not so limited that he has only a few possibilities, he has infinite possibilities. Therefore he does not decide only one possibility to come into being, he decides realm of possibilities to come on the earth to be manifested because there are hundreds and hundreds of possibilities. So he says that there are many possible ways in which possibilities can be combined. For this particular world on which we are living there is a law of compossibility. It is a new concept he introduced in logic – compossibility, not only possibility but compossibility. Events are decided by God to come on the existence, possible can be brought into manifestation if they are compossible. Act will occur in this world if act is compossible with a,b,c,d,e,f,g, as again why if it comes like this it will be compossible only with a and b. So if y is compossible with only a and b it will not occur. God will choose such a compossibility where a largest number of possibilities can occur and they can all be fixed up in a harmony. That is why he spoke of a law of what is called pre–established harmony. According to him all that happens in the world happens because it is all harmony. So according to him the mark of rationality is harmony. It is a very important insight that we are given that a rational person, rationality consists not only perceiving permanence, not only perceiving universality but also perceiving necessity and also for the existence of possibility and contingencies and contingencies happening because they are possible and they are possible because they are compossible and compossible because they are harmonious. Therefore the mark of rationality is to seek for harmony and to discover the harmony, to be rational is to discover the harmony. There is always a harmony according to him the whole world is actually a harmony.