Empiricism

===Phenomenalism===
{{Main|Phenomenalism}}
Most of Hume's followers have disagreed with his conclusion that belief in an external world is ''rationally'' unjustifiable, contending that Hume's own principles implicitly contained the rational justification for such a belief, that is, beyond being content to let the issue rest on human instinct, custom and habit.<ref>Morick, H. (1980), Challenges to Empiricism, Hackett Publishing, Indianapolis, IN.</ref>  According to an extreme empiricist theory known as [[phenomenalism]], anticipated by the arguments of both Hume and George Berkeley, a physical object is a kind of construction out of our experiences.<ref>Marconi, Diego (2004), "Fenomenismo"', in [[Gianni Vattimo]] and Gaetano Chiurazzi (eds.), ''L'Enciclopedia Garzanti di Filosofia'', 3rd edition, Garzanti, Milan, Italy.</ref>  Phenomenalism is the view that physical objects, properties, events (whatever is physical) are reducible to mental objects, properties, events. Ultimately, only mental objects, properties, events, exist—hence the closely related term [[subjective idealism]]. By the phenomenalistic line of thinking, to have a visual experience of a real physical thing is to have an experience of a certain kind of group of experiences.  This type of set of experiences possesses a constancy and coherence that is lacking in the set of experiences of which hallucinations, for example, are a part. As [[John Stuart Mill]] put it in the mid-19th century, matter is the "permanent possibility of sensation".<ref>Mill, J.S., "An Examination of Sir William Rowan Hamilton's Philosophy", in A.J. Ayer and Ramond Winch (eds.), British Empirical Philosophers, Simon and Schuster, New York, NY, 1968.{{page needed|date=December 2013}}</ref>
Mill's empiricism went a significant step beyond Hume in still another respect: in maintaining that induction is necessary for ''all'' meaningful knowledge including mathematics.  As summarized by D.W. Hamlin:

{{quote|[Mill] claimed that mathematical truths were merely very highly confirmed generalizations from experience; mathematical inference, generally conceived as deductive [and ''a priori''] in nature, Mill set down as founded on induction.  Thus, in Mill's philosophy there was no real place for knowledge based on relations of ideas.  In his view logical and mathematical necessity is psychological; we are merely unable to conceive any other possibilities than those that logical and mathematical propositions assert. This is perhaps the most extreme version of empiricism known, but it has not found many defenders.<ref name = MEP2/>}}

Mill's empiricism thus held that knowledge of any kind is not from direct experience but an inductive inference from direct experience.<ref>Wilson, Fred (2005), "John Stuart Mill", in Edward N. Zalta (ed.), Stanford Encyclopedia of Philosophy.</ref>  The problems other philosophers have had with Mill's position center around the following issues: Firstly, Mill's formulation encounters difficulty when it describes what direct experience is by differentiating only between actual and possible sensations.  This misses some key discussion concerning conditions under which such "groups of permanent possibilities of sensation" might exist in the first place.  Berkeley put God in that gap; the phenomenalists, including Mill, essentially left the question unanswered. In the end, lacking an acknowledgement of an aspect of "reality" that goes beyond mere "possibilities of sensation", such a position leads to a version of subjective idealism. Questions of how floor beams continue to support a floor while unobserved, how trees continue to grow while unobserved and untouched by human hands, etc., remain unanswered, and perhaps unanswerable in these terms.<ref name = MEP2/><ref name=MEP6>Macmillan Encyclopedia of Philosophy (1969), "Phenomenalism", vol. 6, p. 131.</ref> Secondly, Mill's formulation leaves open the unsettling possibility that the "gap-filling entities are purely possibilities and not actualities at all".<ref name="MEP6"/>  Thirdly, Mill's position, by calling mathematics merely another species of inductive inference, misapprehends mathematics.  It fails to fully consider the structure and method of [[mathematical science]], the products of which are arrived at through an internally consistent [[deductive reasoning|deductive]] set of procedures which do not, either today or at the time Mill wrote, fall under the agreed meaning of [[inductive reasoning|induction]].<ref name=MEP2>Macmillan Encyclopedia of Philosophy (1969), "Empiricism", vol. 2, p. 503.</ref><ref name="MEP6"/><ref>Macmillan Encyclopedia of Philosophy (1969), "Axiomatic Method", vol. 5, pp. 188–89, 191ff.</ref>

The phenomenalist phase of post-Humean empiricism ended by the 1940s, for by that time it had become obvious that statements about physical things could not be translated into statements about actual and possible sense data.<ref>Bolender, John (1998), "Factual Phenomenalism: A Supervenience Theory"', Sorites, no. 9, pp. 16–31.</ref>  If a physical object statement is to be translatable into a sense-data statement, the former must be at least deducible from the latter. But it came to be realized that there is no finite set of statements about actual and possible sense-data from which we can deduce even a single physical-object statement.  The translating or paraphrasing statement must be couched in terms of normal observers in normal conditions of observation. There is, however, no ''finite'' set of statements that are couched in purely sensory terms and can express the satisfaction of the condition of the presence of a normal observer. According to phenomenalism, to say that a normal observer is present is to make the hypothetical statement that were a doctor to inspect the observer, the observer would appear to the doctor to be normal.  But, of course, the doctor himself must be a normal observer. If we are to specify this doctor's normality in sensory terms, we must make reference to a second doctor who, when inspecting the sense organs of the first doctor, would himself have to have the sense data a normal observer has when inspecting the sense organs of a subject who is a normal observer. And if we are to specify in sensory terms that the second doctor is a normal observer, we must refer to a third doctor, and so on (also see the [[Third Man Argument|third man]]).<ref>Berlin, Isaiah (2004), The Refutation of Phenomenalism, Isaiah Berlin Virtual Library.</ref><ref>{{cite journal |last1=Chisholm |first1=Roderick M. |date=September 9, 1948 |title=The Problem of Empiricism |journal=The Journal of Philosophy |volume=45 |issue=19 |pages=512–17 |jstor=2019108 |doi=10.2307/2019108}}</ref>