Definition of
   a priori






Mental synthesis /coupling, grouping, composition, construction, conjunction/ implies that two or more related (see below) premises are experienced as connected; we note that they somehow are correlated.

We then create a synthesis, a mental connection or association, based on the premises. Depending on how they are experienced as being related, the result of the reasoning becomes a conclusion or a concept. We may also note other results from synthesis like imitation, surprise, joy, and so forth.


As in all reasoning, the premises of the synthesis are ultimately formed through perception, memories from perception or through results from previous reasoning.

As example a certain size interval, an approximately round object, a special taste and texture, certain colours and a special manner of growth may be synthesised to the concept "apple".


Increased credibility


A synthesis that connects several premises becomes more credible than each separate premise:

Nothing so like as eggs; yet no one, on account of this appearing similarity, expects the same taste and relish in all of them. It is only after a long course of uniform experiments in any kind, that we attain a firm reliance and security with regard to a particular event.


Credibility hence increases through synthesis but, as the ultimate premises are always probability arguments, the result of the argument always becomes this type of argument.


How does the brain perform synthesis?


The brain obviously effortlessly performs the art of synthesising between related (see below) premises, i.e. between related perceptions, memories from perceptions and conclusions from previous reasoning.

For example when two phenomena are perceived simultaneously [Stein] we note that the brain creates a connection or association path between them. The connection becomes stronger, or added, when we remember a similar connection from past experience.

When two phenomena are not simultaneously perceived, but still are judged by the brain to be related, it is plausible that the representation of the previous phenomenon is found in the memory (which at this website is called analysis), followed by that a connection is created in a manner resembling that for two simultaneous phenomena.


The result of repeated such processes is a network formed by perceptions and concepts as illustrated below. The image actually shows a social network [Grudz], but is here representing a minute brain based of roughly 100 perceptions (green) and about 250 concepts (red).

Perceptions are combined into concepts that in turn may be used to form more composite concepts.

The image also illustrates the fact that all of our concepts may be seen as directly or indirectly connected.

Perceptions (green) and concepts (red)

Activation / Inhibition

That the brain is connected may result in problems that also may be seen in other feedback networks. Activation of one connection activates other connections that may activate the initial one. The brain would crush into chaos reminding of an epileptic seizure.

This is prevented /inhibited/ by specific or more general processes, e.g. through secretion of inhibiting neurotransmitters.


I suppose that varying grades of inhibition are revealed by differences in focusing between various individuals. Some may follow reasoning in few steps, some in several steps, and some get lost in high-flying association paths when reasoning is to be performed.


Related: Similarity, Contrast, Contiguity


So, what is implied by the term related? The discussion below is a complement to modern opinions about concepts:

Aristotle claimed that similarity, contrariety and/or contiguity create and association, i.e. the experience of a connection [Aristotle]. No better suggestions have been presented since then [e.g. Mill].

... having started in thought from the present or some other, and from something either similar, or contrary, to what we seek, or else from that which is contiguous with it.

Aristotle - On Memory, §2, 451b17

Aristotle's terms are strongly abstracted and include a great amount of less abstracted concepts. "Similarity" may imply e.g. appearance or pronunciation, and "contiguity" may imply e.g. contiguity in time, in space, or in a poem.


The term "contrariety" implies something in direct opposition, but Aristotle probably meant something more general.

The terms "separateness", "incomparable" or "contrasting" probably describe better what Aristotle intended to express.

I therefore use the term "contrast".


David Hume claimed that resemblance, contiguity in time or place, and "cause or effect" create the experience of connection [Hume1]. But he also claimed that the connection between "cause and effect" was created through resemblance [Hume2].

When we perceive a phenomenon, we faster associate to a related phenomenon than to one that is not related. This effect is called priming within psychology [Meyer, Maljkovic, Mayr, Kristjánsson].


Examples of related premises


Synthesis through similarity - Taxonomy

Synthesis based on similarity is seen e.g. during biological concept formation /grouping, systematics, taxonomy creation/. One question is what properties should be used during concept formation.

About the year 1783 Carl Linnaeus grouped plants based on similarities in stamens and pistils. One of his conclusions was that potato and tomato were closely related.

But according to grouping based on other properties like appearance, manner of growth and texture, Linnaeus' conclusion seemed to by improbable and has been discussed frequently through the years.

It was only after analysis of the plants' DNA [Spooner] that Linnaeus' conclusion was established in a very trustworthy manner.


Synthesis through contiguity in time - multimodal perception

The experienced credibility of an occurrence becomes quite significant when the brain simultaneously create many different perceptions from it, i.e. when several senses are stimulated simultaneously by different stimuli (multimodal stimuli).


When I see an apple in my hand, feel it with my hand, and appreciate its odour and taste, I become strongly convinced that the apple actually exists. One of the senses may have committed a mistake, but the probability of that several senses simultaneously do so is very low.


Experience of "the Self"

Our trust in multimodal stimuli is so strong that we easily are misled by artificial correlations, i.e. when two or more senses simultaneously are tricked to report something erroneous to the brain.

An example is the "rubber hand illusion" [Botvinick] when the eyes report that a visible rubber hands is stroked, and the real but invisible hand at the same time reports that it feels being stroked.

An even more fascinating example, with religious and philosophical implications, show the ease of transferring the experience of the "self" to an external object [Ehrsson] by using false correlated perceptions.


The examples clearly show that the experience of the body is formed by multimodal perception that normally is very trustworthy.


Synthesis through similarity - Induction

In criticism of scientific methodology, uncertainty during induction is often claimed to be a important issue. This is therefore discussed separately at the page Induction.


Synthesis through contiguity in time and space - Causal connection

Causal connection, i.e. connection between "cause and effect /result/ , "causing and caused" implies a synthesis between (commonly) two premises, where one is experienced as the cause and the other as the result.

David Hume discussed in an easily understandable manner that causal connections exist and are important, that we know such connections from induction, (which he called "habit") that ultimately is based on perception [Hume3], and that the relation "cause and effect" hence is not "absolutely certain".


Several philosophers claim that Hume expressed that causal connections do not exist, which provide an example of that philosophers can express opinions without empirical evidence.

This is still more remarkable when a prominent rationalistic philosopher, Immanuel Kant, have addressed this question:


The discussion (by Hume about causality) was only about the origin of this concept, not about its indispensability in use

Kant 1783 - Prolegomena 4:259.
Aristotle - On Memory, §2, 451b17.
Botvinick & Cohen - Rubber hands "feel" touch that eyes see, Nature 391 (1998) 756.
Ehrsson - The Experimental Induction of Out-of-Body Experiences, Science 317 (2007) 1048.
Grudz A & Haythornthwaite C - Enabling Community Through Social Media, J Med Internet Res 2013;15(10):e248.
Hume1 1777 - Enquiry, ESB 19, p.24.
Hume2 1777 - Enquiry, ESB 31, p.36.
Hume3 1777 - Enquiry, ESB 22-25, p.26-30.
Kristjánsson & Campana 2010 - Where perception meets memory: A review of repetition priming in visual search tasks, Attention, Perception, & Psychophysics 72 p.5-18.
Maljkovic & Nakayama 1994 - Priming of pop-out: I. Role of features, Memory & Cognition 22, p.657-672
Mayr & Buchner 2007 - Negative Priming as a Memory Phenomenon, J Psychology 215, p.35–51.
Meyer & Schvaneveldt 1971 - Facilitation in Recognizing Pairs Of Words: Evidence of a Dependence Between Retrieval Operations, J Experimental Psychology 90, p.227-234.
Mill, James 1878 - Analysis 2Ed (Longmans) vol.1, p.269.
Spooner et al. 1993 - Chloroplast DNA Evidence for the Interrelationships of Tomatoes, Potatoes, and Pepinos (Solanaceae), Am. J Botany 80, p.676-688.
Stein & Stanford 2008 - Multisensory integration: current issues from the perspective of the single neuron, Nature reviews Neuroscience, 9 p.255-266.