Once, when debating in forum, I was asked what evidence was i.e. what is a definition for evidence ... I admit I was shocked, frankly I had never even given the matter any though before, so I sat back to give the matter some thought.

An "assertion" is something we can say about the universe in which we live or is relevant to some state of that universe. For the purpose of this discussion I am interested only in assertions as they pertain to the real (natural or materialistic) universe.

"Evidence" is anything that may support an assertion or increase its likelihood of being correct. All assessment of evidence is a form of probability, an interpretation, and is dependent upon the observer or interpreter.

In common usage the word "observe" simply means to see, to view something, but in scientific terms an observation (or rather a relevant observation) is a piece of evidence that has been agreed by all relevant parties to be correctly and accurately associated with the assertion which it is claimed to be. It is important to note that the use of the word "observation" in a scientific sense can refer to data that has not been directly observed, for example if the population of the United States is said to be around 350 million it is understood that that is backed up by population data … no single individual is capable of directly observing all 350 million individuals simultaneously.

An observation should be regarded evidence when it is:

• Compatible with the assertion … it is pointless to observe that trees are tall and assert that that is why they are green as the observation has no direct relevance to the assertion.
• Not compatible with other assertions ... it is further pointless to observe that trees are tall and assert that that is why they are green when the (more logical) assertion that chlorophyll (abundant in the leaves of trees) better explains why the tree is green.

It is important to note that a lack of evidence, whilst not obstructing a given assertion, may not be used as a supporting observation for a given assertion i.e. it is not, in scientific terms, evidence.

Assume, for instance, that a respected scientist says there is a very strong case for life on Mars … should people who hear of this necessarily take this as evidence that there is life on Mars? Under normal circumstances people will tend to take the word of such a scientist and assume that what that worthy says is fact however if there is reason to doubt then they might start to ask themselves if that scientist might say that there was life on Mars when he or she had no specific evidence. If such doubt exists the first action would be to question whether that scientists statement actually does constitute evidence … is the scientists sphere of expertise compatible with his or her statement, has that scientist made such statements before and how were they evaluated at that time, has that scientist a hidden agenda or ulterior motive in making such a statement?

Despite the fact that the logic is, itself, self-evident there is a human tendency to ignore the second condition and it is common for individuals to regard as evidence observations which are accepted as compatible with other assertions. Sometimes this is because they simply refuse to consider the alternatives and will, instead, consider only a subset of possible assertions whilst at other times it is simply because they like the observation and assertion in question.

It is also important to understand that the conditions for an observation being acceptable as evidence are entirely independent of the nature of the observation i.e. an observation is valid evidence for a given assertion once verified (agreed to be true), once compatibility with the assertion is established and provided it is not also compatible with others.

When making an assertion about the state of our universe it is important to establish that the observational data used to justify that assertion is both fact (true, verifiable), compatible with the assertion and incompatible with others competing assertions.

For instance, when someone claims that the diversity (observation) around us can only be explained by the actions of deity (assertion) not only is there no observed link between deity and diversity (and indeed no observation of deity) but there exists (in the theory of evolution) a perfectly plausible assertion that is supported by a huge number of observations.

• "What Is Evidence", Yahouda Harpaz