- Summa Logicae by Ockham


William of Ockham (c.1287–1347) was a nominalist philosopher in the High Middle Ages. He argued that universals are simply psychological labels. What really exists are individuals. This contradicted the traditional philosophy of Aquinas which sustained the aristotelian view that essence and existence were distinct. 

Aquinas had achieved a synthesis of the two opposing ideas of the day, both of which were Western philosophy's inheritance: religious faith in divine revelation and aristotelian reason. Aquinas based his unification on faith in a top down rationale; Ockham based his beliefs on reason, a bottom up approach.

Realism has been retroactively applied to the platonic concept of forms. Plato thought that the nature of things lay in the ideas we have of them. This means that their ideal existence is considered more real than their sensory individuality. A horse is not a substance but a copy of the idea 'horseness'. This is a double-world concept in which particulars participate in universals.

Aristotle tended towards a realistic conception of universals and maintained that both universals and particulars exist outside our minds. All particulars have an identical universal property which makes them concrete.

The reality of objects was a point of dissension among philosophers in the scholastic thinking of the Middle Ages. The disagreement centred round Aristotle's Categories. This was known to the Middle Ages through The Isagoge (268–270), an introduction to Aristoteles' work by Porphyry, used for a millenium after he died. He asked several influential questions: 

- Do genera and species exist in nature, or are they products of the mind?

- If they apart from the mind, are they corporeal or incorporeal things?

- Do they exist outside the sensory, or are they realized in the latter?

Realists, like Aquinas, believed in the existence of essences, which were outside the mind. Nominalists, like Ockham, disagreed. This was the problem of universals, which included language.

Realists contended that the 10 aristotelian categories are classifications between existence and terminology. Nominalists claimed that the aristotelian categories were actually only two or three, and based on semantic criteria. Ockham took a rather extreme nominalist position: only particulars exist and the concept of universals is a name of a collection of particulars. (It was the germ of the empirical vision of reality.)

Thomas Aquinas affirmed, following Aristotle, that everything in the world had a purpose (telos), and that God's will conforms to the rational ecosystem of natures. The Thomistic synthesis argued that God's will for the world corresponds to the nature of reality. (This was a causal view of reality.) William of Ockham and other medieval nominalists claimed that there is no independent rational order that guides God's decisions. (This initiated the concept of chance.)

(Realism and nominalism appear in Indian philosophy in the schools of Nyaya-Vaisesika, Jainism, Vedanta and Buddhism. Buddhist Apohavada is nominalist. It also reappears among the French philosophes of the Enlightenment. Voltaire writes Candide as a satire of Leibnitz's determinism.)


The Summa Logicae was written in Latin by William of Ockham between 1323 and 1326. It was probably finished while Ockham was in Avignon where he had been summoned by Pope John XXII on charges of heresy about the doctrine of transubstantiation 

He presents his treatise as a textbook of logic but he also wants to promote his philosophical programme which founded thinking on logic and language. He believes that errors in this thinking derive from the tendency to generate entities according to names, whereas not all names correspond to an entity, a philosophy known as nominalism. It has a famous maxim which expresses that we should not multiply entities more than is necessary, a call for simplification known as Ockham's razor, though he did not use this terminology.

Some regard his enterprise as the beginning of empirical thought in Europe and others claim that it is the anticipation of 20th. century linguistic philosophy.

Ockham organised the work's almost 1,000 pages in three parts following Aristotle's three functions of understanding in his Organon: concepts and their terms; propositions formed by combining terms; argumentation.

Part I deals with the understanding of simple concepts signified by names. It covers Porphry's theory of predicable and aristotelian categories. 

Part II is about the combination of simple concepts into propositions based on Aristotle's De interpretatione. 

Part III explains arguments which are the combination of propositions from several aristotelian works: syllogisms, scientific demonstration, consequences and fallacies.


Ockham´s Razor

"It is vain to do with more what can be done with fewer."

This is the principle of simplicity which states that the simpler theory is probably truer. He borrowed the idea from Aristotle and Aquinas.

Ockham's razor limitations in his search for simplicity applies to hypotheses, not entities. Theories are destined to explain and predict which can be better achieved with less assumptions. His aim was not to arrive at truth but rather to reduce errors. The more hypotheses the more chance of being wrong and every extra hypothesis increases that risk. However, Ockham's avoidance of error led him to question traditional truths. This meant that he gained the reputation as an attacker of the scholastic synthesis of faith and reason.

In A Brief History of Time, Stephen Hawking credits the discovery of quantum mechanics to Ockham’s Razor.


Heraclitus asserted that reality is in flux and nothing remains the same. Taken to its logical conclusion this means that we must be radically sceptical since certainty about anything is impossible. Plato and Augustine disagree that uncertainty is the only certainty. They argue that something remains the same under the perceived variations of reality. They called it the universal essence of things. Humans are different and changeable but have the universal essence of humanity which does not change. A universal essence is a metaphysical reality which determines the invisible structure of things. This is metaphysical realism since they are real though invisible.

Ockham avoids this dilemma of the choice between scepticism and metaphysical realism by arguing that universal essences are mental concepts: nominalism or conceptism. His version of this idea is that universal essences are concepts which arise in our mind when we perceive similarities among things. For him 'humanity' is the expression of perceived relationships between individuals.

"There is no universal outside the mind really existing in individual substances or in the essences of things…. The reason is that everything that is not many things is necessarily one thing in number and consequently a singular thing."


Ockham believes, with Aristotle, that humans are blank slates at birth. We learn by observing qualities in things. Aquinas believed in representationalism in which humans perceive the world through a mental representation. Berkeley took this to an extreme in idealism by saying that the intermediary was God who feeds our mind directly without the need for physical objects.

Ockham avoids idealism through intuitive cognition. It works in 4 steps: sensory cognition; intuitive cognition; recordative cognition; abstractive cognition. The mind is passive in intuitive cognition but objects impinge on our awareness of their existence and this justifies belief in them. Every thing in the world is individual and this is how we perceive it. Brains receive input and organise it by remembering perceptions (recordative cognition) and grouping them (abstractive cognition). 


According to Ockham at birth the mind is a blank slate but it is equipped with a thought system for processing perceptions: a wordless mental language. (This is similar to Chomsky's concept of mentalese.)

The author's interest in avoiding errors led him to argue that many philosophical errors are due to language. Like later 20th. century philosopher Ludwig Wittgenstein, Ockham is motivated by the clarification of confusion in concepts. He rejects the realist conception that thinking of human beings in general leads to the word "humanity" which must be a universal essence within humans. For Ockham the word "humanity" is a habit that allows us to include our experiences of individual humans in an efficient manner. 

The Categories

In Organon Aristotle sustains that there are 10 categories of existing things: substance, quality, quantity, relation, place, time, position, state, action, and passion. However, Ockham uses his razor to interpret Aristotle's categories as only two: substance and quality. The rest are nominal, meaning they do not exist. However, this thesis led directly to Ockham being accused of heresy about the doctrine of transubstantiation, the transformation of bread and wine into the body and blood of Christ. For the author substance and quantity are the same, so a change in the substance means a change in the quantity. This implies that the qualities cannot transfer into a different substance, which is needed in changing bread and wine into body and blood. Despite Ockham's belief in transubstantiation and his attempts to create a logical theory to explain it, he was accused of heresy. 


The author never renounced Catholicism, though his logic questioned certain dogmas. So he embraced fideism, the belief that God is a matter of faith, not reason:

- He believed that theology was not a science. As a convinced empiricist he maintained that all knowledge comes from experience. However, we have no knowledge of God. Ockham's response was hope that after death we can have this knowledge of the divinity.

- The author thought that the Trinity was a contradiction and he demonstrated this through a logical syllogism: the doctrine states that God is the Father and Jesus is God. Therefore Jesus must be the Father. However, the doctrine says that Jesus is not the Father. In conclusion, the dogma maintains that Jesus is and is not the Father. This is contradictory and for Ockham, in his fideist belief, proves that philosophy must not be mixed with theology.

- Aristotle believed that Nature had a purpose (telos) due to a supernatural force. Medieval philosophers found aristotelian thought lent support to the belief in divine providence in which all events are part of God's plan. Ockham found himself generally in agreement with Aristotle, but he is critical of this final cause:

"If I accepted no authority, I would claim that it cannot be proved either from statements known in themselves or from experience that every effect has a final cause."

His statement is hypothetical in order to avoid being accused of openly proclaiming that the universe might have no purpose. (Atheistic existentialism later proposed exactly this idea in the 20th. century.)

The Proofs of God’s Existence

Anselm put forward the ontological proof of God's existence and Aquinas proffered the cosmological proofs. Ockham rejected all such supposed proofs and held his own view based on the impossibility of infinite regress:

- If God does not exist, then there is an infinite regress.

- But infinite regresses are impossible.

- Therefore, God must exist.

Basically Ockham's fideism is a rejection of relying on logical hypotheses, not faith, for belief in God.


For Ockham it is God's will that defines morality because the divinity is the ethical standard. However, since God is omnipotent the divinity can do anything that does not imply contradiction. The divine nature, as conceived by Ockham, is supremely free, so that, contradicting Aquinas, God could command unkindness or any other moral fault:

I reply that hatred, theft, adultery, and the like may involve evil according to the common law, in so far as they are done by someone who is obligated by a divine command to perform the opposite act. As far as everything absolute in these actions is concerned, however, God can perform them without involving any evil. And they can even be performed meritoriously by someone on earth if they should fall under a divine command, just as now the opposite of these, in fact, fall under a divine command.

Free will

Protestant theology traditionally promotes theological determinism: everything humans do is predetermined by God. Catholic theology follows the tradition that God gave human beings free will.

Ockham's view is Catholic and is also influenced by empiricism. He believes in free will because it is an experience. This means that it is central to human nature and since we are made in God's image his conclusion is that it is also in God's nature. 

"To deny every agent this equal or contrary power is to destroy every praise and blame, every council and deliberation, every freedom of the will. Indeed, without it, the will would not make a human being free any more than appetite does an ass."


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