Some philosophers restrict evidence even further, for example, to only conscious, propositional or factive mental states. Restricting evidence to conscious mental states has the implausible consequence that many simple everyday beliefs would be unjustified. This is why it is more common to hold that all kinds of mental states, including stored but currently unconscious beliefs, can act as evidence. Various of the roles played by evidence in reasoning, for example, in explanatory, probabilistic and deductive reasoning, suggest that evidence has to be propositional in nature, i.e. that it is correctly expressed by propositional attitude verbs like "believe" together with a that-clause, like "that something is burning". But it runs counter to the common practice of treating non-propositional sense-experiences, like bodily pains, as evidence. Its defenders sometimes combine it with the view that evidence has to be factive, i.e. that only attitudes towards true propositions constitute evidence. In this view, there is no misleading evidence. The olfactory experience of smoke would count as evidence if it was produced by a fire but not if it was produced by a smoke generator. This position has problems in explaining why it is still rational for the subject to believe that there is a fire even though the olfactory experience cannot be considered evidence.
The traditional view proposes that evidence is empirical if it is constituted by or accessible to sensory experience. This involves experiences arising from the stimulation of the sense organs, like visual or auditory experiences, but the term is often used in a wider sense including memories and introspection. It is usually seen as excluding purely intellectual experiences, like rational insights or intuitions used to justify basic logical or mathematical principles. The terms empirical and observable are closely related and sometimes used as synonyms.
There is an active debate in contemporary philosophy of science as to what should be regarded as observable or empirical in contrast to unobservable or merely theoretical objects. There is general consensus that everyday objects like books or houses are observable since they are accessible via unaided perception, but disagreement starts for objects that are only accessible through aided perception. This includes using telescopes to study distant galaxies, microscopes to study bacteria or using cloud chambers to study positrons. So the question is whether distant galaxies, bacteria or positrons should be regarded as observable or merely theoretical objects. Some even hold that any measurement process of an entity should be considered an observation of this entity. In this sense, the interior of the Sun is observable since neutrinos originating there can be detected. The difficulty with this debate is that there is a continuity of cases going from looking at something with the naked eye, through a window, through a pair of glasses, through a microscope, etc. Because of this continuity, drawing the line between any two adjacent cases seems to be arbitrary. One way to avoid these difficulties is to hold that it is a mistake to identify the empirical with what is observable or sensible. Instead, it has been suggested that empirical evidence can include unobservable entities as long as they are detectable through suitable measurements. A problem with this approach is that it is rather far from the original meaning of "empirical", which contains the reference to experience.
Two central questions for this distinction concern the relevant sense of "experience" and of "dependence". The paradigmatic justification of knowledge a posteriori consists in sensory experience, but other mental phenomena, like memory or introspection, are also usually included in it. But purely intellectual experiences, like rational insights or intuitions used to justify basic logical or mathematical principles, are normally excluded from it. There are different senses in which knowledge may be said to depend on experience. In order to know a proposition, the subject has to be able to entertain this proposition, i.e. possess the relevant concepts. For example, experience is necessary to entertain the proposition "if something is red all over then it is not green all over" because the terms "red" and "green" have to be acquired this way. But the sense of dependence most relevant to empirical evidence concerns the status of justification of a belief. So experience may be needed to acquire the relevant concepts in the example above, but once these concepts are possessed, no further experience providing empirical evidence is needed to know that the proposition is true, which is why it is considered to be justified a priori.
In the philosophy of science, it is sometimes held that there are two sources of empirical evidence: observation and experimentation. The idea behind this distinction is that only experimentation involves manipulation or intervention: phenomena are actively created instead of being passively observed. For example, inserting viral DNA into a bacterium is a form of experimentation while studying planetary orbits through a telescope belongs to mere observation. In these cases, the mutated DNA was actively produced by the biologist while the planetary orbits are independent of the astronomer observing them. Applied to the history of science, it is sometimes held that ancient science is mainly observational while the emphasis on experimentation is only present in modern science and responsible for the scientific revolution. This is sometimes phrased through the expression that modern science actively "puts questions to nature". This distinction also underlies the categorization of sciences into experimental sciences, like physics, and observational sciences, like astronomy. While the distinction is relatively intuitive in paradigmatic cases, it has proven difficult to give a general definition of "intervention" applying to all cases, which is why it is sometimes outright rejected.
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