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Descriptive knowledge, also declarative knowledge or propositional knowledge, is the type of knowledge that is, by its very nature, expressed in declarative sentences or indicative propositions. This distinguishes descriptive knowledge from what is commonly known as "know-how", or procedural knowledge (the knowledge of how, and especially how best, to perform some task), and "knowing of", or knowledge by acquaintance (the knowledge of something's existence).
The difference between knowledge and beliefs is as follows:. A belief is an internal thought or memory which exists in one's mind. Most people accept that for a belief to be knowledge it must be, at least, true and justified. The Gettier problem in philosophy is the question of whether there are any other requirements before a belief can be accepted as knowledge.
Acquiring knowledge 
People have used many methods to try to gain knowledge.
- By reason and logic (perhaps in cooperation with others, using logical argument).
- By mathematical proof.
- By the scientific method.
- By the trial and error method.
- By applying an algorithm.
- By learning from experience.
- By intuition (getting them from the subconscious).
- By an argument from authority, which could be from religious, literary, political, philosophical or scientific authorities.
- By listening to the testimony of witnesses.
- By observing the world in its "natural state"; seeing how the world operates without performing any experiments.
- By acquiring knowledge that is embedded in one's language, culture, or traditions.
- By dialogical enquiry (conversation). See Gadamer, Bohm, Habermas, Freire, on dialogue, learning and knowledge acquisition/negotiation: http://www.infed.org/biblio/b-dialog.htm
- By some claimed form of enlightenment following a period of meditation. (For example, the Buddhist enlightenment known as bodhi)
- By some claimed form of divine illumination, prayer or revelation from a divine agency.
Types of knowledge 
Knowledge can be classified upon a priori knowledge, which is obtained without needing to observe the world, and a posteriori or empirical knowledge, which is only obtained after observing the world or interacting with it in some way.
Often knowledge is gained by combining or extending other knowledge in various ways. Isaac Newton famously wrote: "If I have seen further... it is by standing on the shoulders of giants". (This in order to mollify Robert Hooke) to prevent a dispute.
Inferential knowledge is based on reasoning from facts or from other inferential knowledge such as a theory. Such knowledge may or may not be verifiable by observation or testing. The distinction between factual knowledge and inferential knowledge has been explored by the discipline of general semantics.
Knowledge in various disciplines 
There are many different disciplines that generate beliefs that can be regarded as knowledge. They include science (which generates scientific theories), law (which generates verdicts), history (which generates historical narratives), and mathematics (which generates proofs).
Knowledge in science and engineering 
Scientists attempt to gain knowledge through the scientific method. In this method, scientists start by finding a phenomenon of interest, which generates questions. A scientist then picks a question of interest, and based on previous knowledge, develops a hypothesis. The scientist then designs a controlled experiment which will allow him to test the hypothesis against the real world. He then makes predictions about the outcome of the test, based on the hypothesis.
At this point, the scientist carries out the experiment and compares his predictions with his observations. Assuming that there were no flaws in the experiment, then if they match, this is evidence in favour of the hypothesis. If they do not match, then the hypothesis has been falsified. The next steps are peer review and publication, through which the results are distributed to other scientists.
A hypothesis that has been shown to accurately and reliably predict and characterize some physical phenomenon, and has been sufficiently peer-reviewed and tested, may become a scientific theory. Scientific theories are widely regarded as knowledge, and they are always subject to further revision or review should new data come to light.
To use scientific theories, they must be applied to the specific situation in hand. For example, a civil engineer might use the theory of statics (a branch of physics) to determine whether a bridge will hold up. This is one case where new knowledge is generated from scientific knowledge by specializing it to an individual instance.
The nature of human reasoning dictates that even a sound piece of scientific work might be regarded as incorrect by the scientific community at large. This is exemplified by Dan Shechtman's discovery in solid states for which he was criticised for some time.
Knowledge in history 
The scientific method does not apply to history (or related disciplines, such as archeology), because it is not possible to construct experiments to test theories. Suppose a historian believes that Napoleon would have won the Battle of Waterloo if Gebhard Leberecht von Blücher had arrived an hour later. The historian cannot simply re-run the battle and see what would happen with different starting conditions.
Additionally, the scientific method is essentially the application of the inductive approach to investigation. This approach is entirely appropriate for exploration of the causal world of nature (physics, chemistry, etc.) but not valid for the teleological social sciences, which includes history. There are no constants in human relations, only unmeasurable and inconstant subjective valuations. Electrons always behave the same way under the same conditions, but humans do not—different people seem to react differently and the same person seems to or might react differently at different moments in time. Thus, it appears that only spurious inferences can be drawn from repeated observations of human behavior. It might be observed that most humans prefer wealth to poverty or life to death, but it might be invalid to infer any universal law of human behavior from this.
Historians often generate different interpretations of the same event, even when reading the same primary sources, and these interpretations are always subject to revision by other historians. This is because, as a social scientist, the historian must constantly make subjective judgements of relevance in trying to interpret historical events.
Situated knowledge 
Situated knowledge is knowledge specific to a particular situation. Imagine two very similar breeds of mushroom, which grow on either side of a mountain, one nutritious, one poisonous. Relying on knowledge from one side of an ecological boundary, after crossing to the other, may lead to starving rather than eating perfectly healthy food near at hand, or to poisoning oneself by mistake.
Some methods of generating knowledge, such as trial and error, or learning from experience, tend to create highly situational knowledge. One of the main benefits of the scientific method is that the theories it generates are much less situational than knowledge gained by other methods.
Situational knowledge is often embedded in language, culture, or traditions. Critics of cultural imperialism argue that the rise of a global monoculture causes a loss of local knowledge.
What constitutes knowledge, certainty and truth are controversial issues. These issues are debated by philosophers, social scientists, and historians. Ludwig Wittgenstein wrote "On Certainty" - aphorisms on these concepts - exploring relationships between knowledge and certainty. A thread of his concern has become an entire field, the philosophy of action.
A number of problems exist, that arise when defining knowledge or truth, including issues with objectivity, adequacy and limits to justification. Beliefs are also very problematic not least because they are either true or false, and therefore cannot be adequately described by conventional logic. An action likewise can be taken or not, but there is the troubling idea of an "event" is, an action taken by nobody, or nobody who you can blame.
Non-scientific methods 
Several groups, most notably the postmodernists and social constructivists, hold that science does not actually tell us about the physical world in which they live. They hold that the world cannot be understood by science, but rather by religious revelations, mystical experience, or literary deconstructionism.
Practical limits for obtaining knowledge 
What we hold to be knowledge is often derived by a combination of reason from either traditional, authoritative, or scientific sources. Many times such knowledge is not verifiable; sometimes the process of testing is prohibitively dangerous or expensive. For instance, some physics theories about the nature of the universe, such as string-theory, require the construction of testing equipment currently beyond our technology. Since such theories are in principle subject to verification or refutation, they are scientific; since they are not proven experimentally, they are not considered certain knowledge. Rather, in such cases we have certain knowledge only of the theory, but not of what the theory describes.
"Of the three ways in which men think that they acquire knowledge of things—authority, reasoning, and experience—only the last is effective and able to bring peace to the intellect." (Roger Bacon, English alchemist, astrologer, philosopher and a major progenitor of modern science.