When one has a true belief in something then that means that the belief is irrefutable and undoubted. True belief is not open to challenge in the believer's mind. There are many reasons for this, ranging from the completely objective

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In order to facilitate the consideration of the question asked it would be useful to expand on this one definition of knowledge i.e. that knowledge is true belief based on strong evidence. When one has a true belief in something then that means that the belief is irrefutable and undoubted. True belief is not open to challenge in the believer’s mind. There are many reasons for this, ranging from the completely objective to the completely subjective, and somewhere in this aforementioned range strong evidence would be a clear milestone in the acceptability of a belief. One definition of evidence is “something that gives a sign or proof of the existence or truth of something, or that helps somebody to come to a particular conclusion”. However strong evidence implies that there is a possibility that the evidence is not infallible but rather that it is almost certainly true. It is this latter consideration about the strength of evidence that leads one to the conclusion that the limit marking strong evidence is not static but rather becomes “elastic”, depending on various factors and the particular area of knowledge, and this has to be borne in mind when establishing the limit of strong evidence.

A counter argument to the above would be that once empirical scientific evidence is insufficient to support a true belief, then it is at this exact point that the limit of strong evidence has been reached. However, by looking at the various areas and ways of knowledge it can be seen that establishing the limit where strong evidence is achieved is not so clear-cut. One can argue that in the spectrum between complete objectivity and complete subjectivity, the sciences as an area of knowledge lies in the objective range because conclusions in the scientific world are based upon facts established through empirical studies and their outcome. The beliefs based on mathematics and the natural sciences can be experienced and verified directly in the natural world. This makes the evidence “strong”. For example there is very strong evidence to suggest that long term smoking is generally dangerous to your health and can cause lung disease. The evidence can be clearly seen when looking at the dissected lung of a smoker who has died from lung cancer and comparing it to that of a non-smoker. However, what about long term chain-smokers who do not show any sign of lung disease? This is an example where the evidence is strong but there is some doubt. A limit however can be established whereby the strength of the evidence is such that we can believe that smoking causes damage to the lungs. This limit is established by clinical observation and comparison between smokers and non-smokers, but also through non-scientific ways of knowledge such as intuition and feeling. Intuition tells us that smoke being inhaled into the lungs might be harmful and this is supported by feelings such as coarse coughing which follows the inhalation, especially felt by first-time smokers.

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Further along the spectrum away from complete objectivity, the fact that the limit establishing strong evidence is not static can be perceived in other areas of knowledge such as the human sciences, history, arts and ethics. In psychology, a true belief might require evidence based on more subjective input. This belief would have to be corroborated through multiple and continuous verifications. For example, does a deprived childhood make somebody more or less ambitious in later life than a normal childhood? The limit of strong evidence in this example, and indeed in this area of knowledge, can probably never be accurately pin-pointed due to the influence of various other factors such as the political environment and the degree of social mobility. Another, more specific example, concerns the so-called Gulf War Syndrome suffered by a number of soldiers who served in the first Gulf war in 1991. Even to this day it has been difficult to ascertain whether the symptoms being experienced by the affected soldiers are the results of exposure to chemical agents or the psychological effects of modern warfare or a combination of both. Thus one can see that the limit signifying strong evidence is clearly elastic even in the scientific fields. The strength of evidence in the case of smoking is more easily communicable as a true belief because of the physical nature of the evidence, whereas psychology involves more subjective personal and interpretative analysis of the evidence, which can be easily disputed and therefore establishing a limit has to be proven to be valid for the majority of people.

Similarly, historical events provide strong evidence on which to base a true belief, however even historical events are open to interpretation and true beliefs concerning them can be challenged. A good example would be the Israeli - Palestinian conflict, where one would think that one could easily determine the correctness of land distribution based on the history of the region and its peoples. However, as is well known the conflict has now escalated to such a degree and dimension where history appears to be a secondary consideration. Thus historical evidence as a basis of true belief has been weakened and other factors influence the establishment of a limit in the formation of a true belief. As one moves up the scale of subjectivity towards the arts and ethics, strong evidence on which to base a belief becomes even more elusive. Is the evidence really strong enough to determine that Picasso was a great artist? (many people think they can draw and paint just as well as Picasso did) or how does one believe from strong evidence, if any, that it is unethical to steal a loaf of bread even when one is dying of hunger?

Particularly where one does not have direct personal experience on which to base a belief, one has to rely on secondary evidence which may or may not be strong enough. For example, from my personal experience living in Greece, I was told by a taxi driver, when struggling to put the seatbelt on in his very old taxi, that it was not necessary to put my seatbelt on since he was a good driver. However, I had a very strong and true belief that putting on a seatbelt was a necessary safety measure. What made the evidence strong enough for me to believe this, since I had never personally been in any car accident, with or without a seatbelt? The evidence was formed by various factors: second hand knowledge of the outcome of car accidents (newspaper reports), the possible fatal consequence of my not wearing a seatbelt, the probability that the taxi driver was wrong (very likely considering the extensive damage to the chassis of this particular taxi) and the fact that most people believe that wearing a seatbelt will reduce the risk of severe injury in a car accident. On the other hand, in another personal experience, the evidence was not strong enough for me to believe something my mother held as a true belief. Before a trip to downtown Frankfurt with my friends my mother told me not to take all my money with me because I might lose it or have it stolen. I did not share this belief because the evidence supporting it was not strong enough in my view. It was not strong enough because I lacked personal experience of having my money stolen, the probability in my opinion was very low with so many of my friends around me, the consequence of losing my money was not so severe and finally the majority (my friends) did not believe there was any danger in losing my money. Therefore the limit of strong evidence had not been reached.

Clearly, moving away from the objectively scientific determination of what constitutes strong evidence requires more factors to be taken into consideration in order to establish the limit where the evidence is strong enough to support a true belief. Such factors can be personal experience or second hand knowledge, the possible consequence of a belief, the probability of a belief being true and whether or not the belief is shared by the majority, the latter which can also be culturally determined. Moreover, the fact that the strength of evidence has a direct correlation to its objectivity means that there can be no absolute steadfast limit where evidence can be judged as strong enough, but rather this limit is variable depending on the area of knowledge.

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One Definition of Knowledge is true belief based on strong evidence. What makes evidence “strong” enough and how can this limit be established?

8. August 2004, http://encarta.msn.com/dictionary_/evidence.html