What is Philosophy
What is Philosophy
of Science
What is Verification
and Falsification
about Science
Science according to
Scientific method
Paradigm and
paradigm shift
Karl Popper -
Logic and status
Consequences of Popper's theses
Alternative Science
Chalmers: What is this
thing called Science?
Epistemology -
induction deduction

Theory of knowledge

Theory of knowledge or Epistemology discusses properties of the term "knowledge" and how knowledge may be created.

A summary of the discussion is: "Do we know anything?" and "Can we prove that we know anything?". The summary continues at the bottom of this page.

Three major, intermixed, views exist:
•  Empiricism
•  Rationalism
•  Skepticism

Existence of knowledge is a core question within philosophy. The discussion indirectly concerns existence of concepts like truth and reliability and if we can decide if anything exists at all.


What is knowledge?

The word "knowledge" is related to terms like "knowing" and "truth".

Within philosophy the term "knowledge" is used in at least three meanings. When the term is used, one factor should be considered important: To define which meaning we give to the term "knowledge" when we use it.

Knowledge - eternal truth / absolutely certain knowledge

What we call knowledge has during previous times varied. Today, we cannot with absolute certainty express what we tomorrow, or in 100 years, will mean with the term "knowledge". Within philosophy is is generally acknowledged that our source of knowledge - our sensory experiences and the conclusions drawn from them - are not representing "absolutely certain knowledge".

It is hence not possible to claim that the "truths" of today will be "true" also tomorrow, even if the "truth" may persist for a long time / always.

Neither is it possible to claim that all "truths" of today will be regarded as "untrue" tomorrow.

It hence appears to be impossible to prove the existence, or non-existence, of knowledge in the meaning of "absolutely certain knowledge".

  More about that "absolutely certain knowledge" cannot be shown to exist  

Knowledge - personal experience

We all experience that we learn things in life. As our lives are different, knowledge according to this view is individual, and differences in what represents knowledge exist.

Knowledge - common experience

There are many examples of groups of people claiming that their common opinions represent knowledge. In a group the claims that the knowledge is correct are strengthened, in spite of that other groups may claim other types of knowledge.

A large part of what we call knowledge, e.g. dogmatic doctrines and science, belongs to this category.

A major difference between dogmatic doctrines and science is that the claimed observations and conclusions that provide the ground for dogmatic systems can not or may not be questioned, while observations and conclusions within science are continuously re-evaluated.

Knowledge - common experience under continuous revisions

Knowledge may also be said to consist of such observations and conclusions that many persons openly investigate, revise, alter and maybe agree about. This type of knowledge is an adaptation to that "eternal truth" cannot be shown to exist.

For this knowledge to be trustworthy, it is important how it is spread and revised:

•  What type of arguments can be used?
•  How can openness be encouraged?

From the answers to such questions, something that reminds about what we today call scientific method will probably be created.


"Knowledge" dependent or independent on experience

As discussed above, the term "knowledge" is not easy to define, and within philosophy this invites to speculation.

A question discussed since Greek antiquity is whether "knowledge" should be based by observations or if we may reach "knowledge" using only logical reasoning, independent of observations.

Aristotle stressed observation together with logical reasoning (deduction), while Plato discussed that our observations cannot be proven to represent a kind of reality. And so it has continued through history of philosophy.

During the Enlightenment the terms became more defined and e.g. David Hume called the two groups, from which our reasonings are built, for "matter of fact" and "relations of ideas".

Reasoning based on "matter of fact" were called probability arguments. Today western philosophers agree about that our experience about the world are based on probability arguments.


"Knowledge" a priori and a posteriori

Without defining the term "knowledge" (see More about that "absolutely certain knowledge" cannot be shown to exist above), Immanuel Kant discussed these topics in terms of "a priori" ("knowledge" that can be justified without referring to observations) and "a posteriori" ("knowledge" that requires observations for justification).

It is agreed that tautologies and other logical operations ("analytic arguments") are performed a priori. But does arguments in addition to these ("synthetic arguments") exist that also can be said to be a priori, or expressed differently: Does arguments a priori about our experienced reality exist? Without such arguments, the relevance of pure analytic philosophy (armchair philosophy) concerning the world can be seriously questioned.

Using vague definitions, Kant claimed existence of "synthetic a priori" "knowledge", i.e. propositions concerning the world that is not based on experience. This has been challenged, and no distinct examples of such "knowledge" have been demonstrated.


Are inherited abilities examples of synthetic a priori "knowledge"

We use logic arguments, or "relations of ideas" at every moment we are awake. But does any experience about the world exist, that could be meaningful to call
a priori? The answer seems to be "No".

The closest concept I can imagine is inherited abilities.

More about inherited abilities


Induction - to draw conclusions from observations

Definition of induction:


The process of inferring a general law from the observation of particular instances


(Oxford English Dictionary).


Short version: Inferring from particulars to generals


A philosophic attitude that argues for the use of induction is called empiricism.

Induction implies that repeated similar observations create expectations of that additional similar observations under similar conditions will yield similar results, and that they therefore represent something general.

Or in everyday language: When we note something several times, we believe that it is true.

We use induction every day, possibly in every moment we are awaken. When we wake up in the morning, we expect the world to look approximately similar to what is was in the evening before. We believe that we should look approximately the same and that the floor remains. Induction is the base for all our experience.

Philosophical problem

A philosophical problem with induction is that the reasoning (that many similar observations may represent something general) is not logically valid.

Neither induction nor any other method will give rise to "absolutely certain knowledge". Regardless of how many observations we have performed that support a hypothesis, we cannot be sure that the hypothesis is "true". This was discussed e.g. by Sextus Empiricus about 200 AD.

    More about induction  




An illustration of the term induction was given by Bertrand Russell in "History of Western Philosophy", in the chapter about Francis Bacon:


There was once upon a time a census officer who had to record
the names of all householders in a certain Welsh village.
The first that he questioned was called William Williams;
so were the second, third, fourth, ...

At last he said to himself:
"This is tedious; evidently they are all called William Williams.
I shall put them down so and take a holiday."

But he was wrong; there was just one whose name was
John Jones. This shows that we may go astray if we trust
too implicitly in induction by simple enumeration.


In my opinion scientific methodology decreases the insecurity inherent in induction. As example a scientific treatment of Russell´s illustration (above) may look like this:

• The census officer publishes that all were named William Williams and how he reached this conclusion.

• Somebody reads this and notes that the work is not complete. She checks the church books and finds one John Jones. She reports this, how she arrived to this result and also describes the earlier work by the census officer.

• A person in the village reads this second report and knows that the neighbor, a recent immigrant, is called Sven Svensson. This is reported as a "letter" to a journal. This "letter" contains references to the two previous reports.

By this method, using induction and verification, a gradually more refined description of the householders in the village is obtained.



Deduction, Rationalism

Deduction - to infer conclusions by logical reasoning

Definition of deduction:


The process of drawing a conclusion from a principle already known or assumed


(Oxford English Dictionary).


Short version: Inferring from generals to particulars


A philosophic attitude that argues for the use of deduction is called rationalism. It is popular within philosophy and is sometimes called "armchair philosophy".

Deductive arguments concerning our perceived world may be defined as

Deductive arguments about our perceived reality
create relations between observation results

The link to the definition above is provided by that "a principle already known or assumed" are based on observation results.

Philosophical problem

A problem with rationalism is that strict deduction, without references to observations, cannot give anything of interest regarding our perceived reality. Differently expressed are arguments that are synthetic a priori not demonstrated to exist. This was discussed in detail by David Hume already at the eighteenth century.

A form of rationalism that is possible to apply while describing our perceived reality therefore must include empiricism.

    More about deduction  


The logical impossibility of strict rationalism was also discussed by Bertrand Russell in his book "History of Western Philosophy":


German philosophers, from Kant to Hegel,
had not assimilated Hume's arguments.
I say this deliberately, in spite of the belief which many philosophers share with Kant, that his "Critique of Pure Reason" answered Hume.

In fact, these philosophers,
at least Kant and Hegel,
represent a pre-Humian type of rationalism,
and can be refuted by Humian arguments.




Skepticism - we cannot know anything with certainty








Skepticism (or Scepticism) has existed since the antique Greece. The relevance of the concept Skepticism is coupled to the definition of the concept of knowledge. See the part "What is knowledge?" near the top of this page.

The word skepticism comes from the Greek word for "to consider". It implies that an issue should be investigated before arguing about it. Skepticism is coupled to empiricism and rejects dogmatic and often deductive reasoning.

Skepticism, like all other philosophies, exists in varying degrees:


When drawn to its extremes Skepticism is called Pyrrhonism.

David Hume is considered as a leading skeptic. He writes about Pyrrhonism in Enquiry concerning Human Understanding (1777) page 159:


These principles may flourish and triumph in the schools where it is, indeed, difficult if not impossible to refute them. But as soon as they leave the shade, and
by the presence of the real objects, which actuate our passions and sentiments, are put in opposition to the more powerful principles of our nature, they vanish like smoke, and leave the most determined skeptic in the same condition as other mortals.


Mitigated Skepticism

During later stages of Plato´s Academy a mitigated, or Academic, Skepticism rose. It argued that some conclusions about reality are more probable than other. This Skepticism was advocated e.g. by David Hume and is the philosophy behind scientific methodology.

Erroneously used Skepticism

In my opinion, references to Skepticism are often used erroneously.

During arguing, leaning on support from Skepticism, it is sometimes claimed that "statement A is not true", and then "because A is not true, then statement B must be so".

A common example from philosophy is that as induction cannot give us "absolutely certain knowledge", deduction must do so.

Supposedly skeptic statements like "Knowledge does not exist", "We can never know anything" or "All is relative" represent contradictions as they, in spite of their content, claim to represent some type of knowledge.



Science /

Science is not the same as "knowledge":

The term "knowledge" may imply several concepts as discussed in the part "What is knowledge?" near the top of this page.

Science consists of results from an activity, a method of work, during which observations or conclusions from observations are reported in a manner that admits of examinations. This adds to the reliability of a scientific proposition, compared to a proposition that is not based on observations.

Science is hence a concept quite different from "knowledge". The two terms are sometimes confused, probably because many scientific publications are regarded as credible descriptions of our perceived world.

Criticism of science using arguments that rightly should be concerned with the concept "knowledge" is, according to my opinion, either deliberately erroneous or a testimony of unawareness about epistemology. I view the scientific method as an answer to our obstacles with the concept "absolutely certain knowledge".




We use induction, deduction and skeptic arguments

As stated above, we cannot prove presence, or absence of anything using philosophically strict arguments. We cannot strictly prove the presence or absence of a stone, nor the existence, or non-existence, of knowledge.

We use induction

But we do not seriously believe that the world will change during the time it takes to twinkle. We dare to take a step and put down our foot, without fear that the ground have disappeared.

Hence we apply induction in our lives, maybe in every conscious moment. We do that in spite of that we know that "knowledge" about the world cannot be proven using strict philosophical arguments.

We use deduction

We probably always create relations between observations. Similarities and dissimilarities are judged by our thoughts. Does the food smell as it should? Do I have the strength to move the heavy stone? Do I want to talk to this angry person?

We are Skeptics

If we reflect, we may sense that we are not entirely certain in our beliefs. Some are more certain, others are less. We believe that the world will exist tomorrow, but we are maybe less certain about that all types of meat are only nutritious.

We have to live with our "truths", even when we cannot prove them to be "absolutely certain knowledge".




Summary of more than 2000 years of epistemology

•  Do we know anything? Probably.

•  Can we prove that we know anything? No, due to the definition of the term "know" (but this does not imply that we know nothing).

•  Can we live with that we cannot prove that that we know anything? Obviously.



Much later than I wrote this summary, I became aware of that David Hume, in his Enquiry formulated the last point in a more poetic manner (p. 160):

...who must act and reason and believe; though they are not able, by their most diligent enquiry, to satisfy themselves concerning the foundation of these operations, or to remove the objections, which may be raised against them.


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