Empiricism is the knowledge set that develops from sense experience or evidence. Empiricism approves those parts of scientific knowledge that has evidence or derived via experiments.
In the empiricism language, established scientific theory is regarded as empirically adequate. Empirically adequate means - what the observable phenomena states is true. When Galileo tried to prove his heliocentrism concept, Cardinal Bellarmin objection was not regarding rotation of earth around sun, but was regarding the heliocentrism itself; because it was an unobserved prediction.  In early time when planetary orbits were just unobservable concepts, Aristotelian and Ptolemaic ideas were disputed by philosophers. Aristotle suggested a mechanism about crystalline spherical concepts (planets and orbits) which states that planet regularly moves and keeps a fixed distance from one another. Ptolemaic had an idea of epicycles which stated about observed planetary motion. But as they were hypothesis based on idea & calculation, not observable, they were refuted by some philosophers. But there were philosophers who believed in epicycles and crystalline sphere model proposed by astronomers that time, considering that they were trying to organize reasons behind observations. 
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While empiricism believes on experiments, realism on other hand believes that aspire of science is to find the reason behind a considered fact. Also well confirmed scientific theory is considered as truth in realism. Scientific realists believe that successful scientific research produces knowledge and it is not necessary for such knowledge that relevant phenomena should be observable. A relevant example: students read a well known chemistry textbook believing the claimed truths that the book demands about existence of from molecular to sub atomic level particles, energy levels etc.
Readers are not experimenting themselves or observe, rather they accept on their Â´face value Â´ and what the well known scientists/researchers have stated. This is why Galileo may have tried to progress with his heliocentrism concept considering that heliocentrism is true, which Cardinal Bellarmin protested.
Logical Empiricists were successors of logical positivism, who were a group of philosophers also known as Vienna circle. They insisted that claiming something empirical, it has to be verified by observation. They faced two type of scientific activity on role of observation, generalizing the observable incidents such as motion of bodies or relation between pressure and gas etc. Scientists developed scientific theory empirically which explained the observational consequences by predicting, compiling and researching data gathered from observations, for example about forces acting on body, kinetic energy etc. Logical empiricists tried to put a demarcation line between the directly observable and unobservable theoretical assumptions based on observations. Two aspects of theoretical-observational distinction are ontological and terminological concept. Ontology is a distinct between perceives and unperceived outcomes. Terminological distinction deals with vocabulary and the unique theoretical terms like atom, molecule, electromagnetic field etc. and empirically speaking others as observational items. Logical empiricists were more or less cynical about the theoretical derivation from unobservable phenomenon or stated that as partially meaningful. (Curd & Cover 1998, p.1227). 
Constructive empiricism focuses on aim of science and not individual aim. Van Fraussen differentiates terminologies to scientific agnostic and scientific Gnostic. (1998, 213). He calls scientific agnostic to those who considers science as empirically adequate but not concerned with the truth or false. Those who want to distinguish scientific theory acceptance between true and false, he termed them Gnostic. Scientific realists considers gnostic as true understandable audience of science but not agnostic. While constructive empiricists believe gnostics may believe or not in scientific enterprise but they adopt the belief regarding involvement and requirement for goal(Van Fraassen 1998, 213-214) 
View of Maxwell and Feyeraband on observable and unobservable aspects.
Maxwell tried to express his view and logics against anti realism about observable and unobservable objects. For clear understanding he gave a fictional example: A scientist Jones considers about mechanism by which disease transfers from one person to another. Considering available observable disease carrier such as lice, Jones concluded that infectious diseases are transmitted by bugs or micro-organisms which mostly are beyond visibility. Depending on the observed facts he proposed preventive mechanism to stop disease transmission transmitted through unobserved micro-organisms. Questions arose from other philosophers that how he analyzed something which was unobservable to him. Invention of microscopes later showed that different organisms are carrier of different disease; apparently the concept which was drawn from observable fact came almost true for unobservable fact. There were mixed responses from philosophers, which can be summarized as:
- Some philosophers insisted that claims about physical objects should be transformable into claims about directly distinguishable sense data.
- Jone's crobes were not unobservable since the impossibility of finding them were not said.
- Microorganisms were not observable and such carriers were merely physical particles.
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For (a) Maxwell does not overlook the importance of the observable phenomena which is considered the ultimate ground for proof but on the other hand he claims just sensory impressions are not the basic unit of scientific discourse but a fast decidable statement is. Maxwell argues that depending on quickly decidable sentences, how one can quickly decide the truth and false about a phenomenon is itself a scientific theoretic question; answer may be observational data as hypothesis of a theory; but such issues are misty.
Pont (b) says crobes were not unobservable. Maxwell protested such view of antirealists about unobservability in principal. Maxwell states that there is no significant philosophical demarcation criterion between observable and unobservable.
While discussing (c), to show the opaque idea about observabilty and unobservability, Maxwell pointed towards a quote from Gustav Bergmann, "..even stars and microscopic objects are not physical in a literal sense" for when we look through our apparatus all we see "is a patch of colour which creeps through the field like a shadow over the wall" (Curd & Cover 1998, p. 1229-1230) Maxwell wonders about the idea of direct observation among anti realists. According to him, for understanding that it is raining we do not need to look at the rain directly. Also he points out that at particle-level, particles are at transition stage; so there is a transition between observable and unobservable shape. Objects watched through microscopes are as real as objects seen through bare eyes. Maxwell came to two conclusions, firstly, there is no established criterion to draw a non arbitrary line between observable and theoretical context. Secondly, there is no ontological significance between observable and theoretical shown by transition of particles. (Curd & Cover 1998, p.1230)
Van Frassen, another philosopher, agrees with Maxwell that at the end statements can be theory laden to some extent, but he disagrees that it has something to do with scientic realism. He also disagrees with Maxwell's statement- that no clear distinction between observable and unobservable phenomena. According to Van Frassen, constructive empiricists accept a theory T when they consider it empirically adequate; they can distinguish between the T statements that are about observable and which are not.
Maxwell stated three arguments regarding realism:
- There are observable objects about which we must be realists.
- No arbitrary lines can be drawn between observable and unobservable objects
- Therefore, we are not warranted in claiming that there is a class of unobservable objects about which we may be antirealists." (Curd & Cover 1998, p.1235)
Van Fraassen argues with most of the conclusions from above statements. For showing the stand about unobservability, he used cloud chamber as an example. Here, charged particles leave a visible trail though those particles are not visible themselves, so observation is a fuzzy prediction.
Some objects are unobservable due to natural limitations. Van Frassen exemplifies that Empire State Building cannot be considered portable because some monster can move these.
For last argument, Van Fraassen says observable and unobservable distinction plays role in philosophy. Empirically adequate statements of constructive empiricists are based on observational facts. Theory is accepted only in observational phenomenon. Hence what the theory says about observability decides realism and antirealism. (Curd & Cover 1998, p.1236)
Once more observable and unobservable distinction
Among empiricists, constructive empiricists considered macroscopic elements as observable, cause they consider they could see those if they were at that dimension. (Paul Churchland 1985, 39-40) . Van Fraassen replies distinction is related to epistemic consideration. Empiricist considers experience as trusted supply of information and experience influences the epistemic attitude. (Van Fraassen, 1998, 258)
Churchland criticizes that constructive empiricists have acknowledged macroscopic objects but not virus as their part of ontology (Churchland 1985, 44-45) .
Van Frussen said on this behalf, that it will be measured when humanoids will be considered as their part of epistemic community. If their epistemic community is expanded to that level they will admit virus as observable else not. (Van Fraassen, 1998, 256-257)
Ian Hacking also put his argument regarding observable and unobservable phenomenon. He considers a machine that produces grids of same shapes but different sizes. Some grids are within human vision range and some made smaller by machine to microscopic level, nevertheless the size is-shape is maintained same. Under microscope it was confirmed that shape of grids within and beyond human vision range are same. Ian Hacking concluded that as setup was reliable and process was accurate, result of the unobservable grids are expected one, so sometimes anti realists should believe in unobservable phenomena. (Ian Hacking 1985, 146-147)
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The empiricists do not consider what is beyond observable by unaided eye, but this time as result is pre determined and confirmed about unobservable particles. It was curious to know what their speech is.
Van Fraassen(1958,298) pointed Hacking's argument as unnecessary supposition, stating that the grid was made to be that way and the argument states unstated premises which constructive empiricists denies. (Van Fraassen1958, 298)
Contemporary Empirisism View
Popper introduced some plausible and simple ideas which are of one of the de facto of contemporary empiricism.
T and T' are taken as two different scientific theories.
Where T stands for reducing or explaining theory and TÂ´ stands for reduced or explained theory. DÂ´ is domain with conditions specified.
T' is deduced from T, maintaining the initial conditions specified within domain DÂ´ where TÂ´ is applicable. Explanation of explaining theory T inside DÂ´ must be compatible with TÂ´. Meaning of TÂ´ should be unaltered from original.
- Only such theories are then admissible in a given domain which either contain the theories already used in this domain, or which are at least consistent with them inside the domain; and
- meaning will have to be invariant with respect to scientific progress; that is, all future theories will have to be phrased in such a manner that their use in explanations does not affect what is said by the theories, or factual reports to be explained "
Feyeraband call these consistency conditions and condition of meaning invariance. He also states that these works in restrictive conditions. Both of the above conditions are in contrast with actual science in some cases.
Problem with consistency condition: Galileo's law of free fall and Newton theory are inconsistent to each other. Galileo's law states that acceleration of free fall remains constant. But Newton's theory states that deceleration takes place from surface of the Earth to Earth-centre.
Actual science also violates the meaning invariance statements. According to classical law:
- Wikipedia-the free encyclopedia, Empiricism,[online] (Updated 20 March 2010) Available at: http://en.wikipedia.org/wiki/Empiricism[Accessed 18 March 2010]
- S. Psillos and M. Curd (eds.), 2008,The Routledge Companion to the Philosophy of Science, Empirisism, 2008 [internet], (published 2008) Available at: http://philosophy.wisc.edu/sober/Empiricism%20oct%20069.pdf [Accessed 18 March 2010]
- Google Books, Science teaching-The role of history and philosophy of science, (author: Michael R. Mathews, Publisher Routledge, 1994), [internet] Available at http://books.google.se/books?id=qnwzRqh5jFMC&pg=PA164&lpg=PA164&dq=galileo+empiricist+or+realist&source=bl&ots=3qW_zzkW86&sig=vIcQT2C-YOkATzQ1By9iH69Ngyg&hl=en&ei=dbehS72lIaaknQP_p-n5CQ&sa=X&oi=book_result&ct=result&resnum=3&ved=0CAsQ6AEwAg#v=onepage&q=galileo%20empiricist%20or%20realist&f=false [Accessed 17 March 2010]
- Scientific Realism-Stanford Encyclopaedia of Philosophy, (Published June 12 2002), [Online] Available at: http://plato.stanford.edu/entries/scientific-realism/, scientific realism [Accessed 19thMar2010]
- Philosophy of Science-The central issue, Martin Curd & J.A Cover, 1998, W.W Norton & Company Ltd.
- Constructive Empiricism -Stanford Encyclopaedia of Philosophy, (Published Oct 1 2008), [Online] Available at: http://plato.stanford.edu/entries/constructive-empiricism/ [Accessed 19thMar2010]