The idea that meanings and concepts are not independent of the processes through which they are constructed must be definitely accepted. It is well-known that the problem of incommensurability emerges from the consideration of the dynamic aspects of scientific enterprise: I will show how it is just this dynamic character that can solve (or weaken) it or at least define its contours.

My contribution aims to introduce the distinction, not previously analyzed, between two kinds of creative abductive reasoning, theoretical and manipulative, in order to provide an integrated framework to explain some of the main aspects of both incommensurability and commensurability effects engendered by the practice of science. The distinction appears to be extremely convenient: incommensurability will be viewed as the result of the highest cases of theoretical abduction (that is the cases of radical innovation), and, at the same time, commensurability will be recovered showing the role of the so-called model-based abduction.

Moreover, manipulative abduction will show how we can find methods of comparability at the experimental stage, where the recent epistemological tradition has settled the most negative effects of theory-ladeness.

Finally, I will illustrate some interesting relationships between the incommensurability thesis and conventionalism.

Many reasoning conclusions that do not proceed in a deductive manner are abductive. Hence, abduction is the process of inferring certain facts and/or laws that render some sentences plausible, that explain some phenomenon or observation; it is the process of reasoning in which explanatory hypotheses are formed and evaluated. I have developed with others an epistemological model of medical reasoning, called the Select and Test Model (ST-MODEL) which can be described in terms of the classical notions of abduction, deduction and induction: it describes the different roles played by such basic inference types in developing various kinds of medical reasoning (diagnosis, therapy planning, monitoring) but can be regarded also as an illustration of scientific theory change. There are two main epistemological meanings of the word abduction: 1) abduction that only generates plausible hypotheses (selective or creative) and 2) abduction considered as inference to the best explanation, that alsoevaluates hypotheses. All we can expect of our "selective" abduction, is that it tends to produce hypotheses that have some chance of turning out to be the best explanation. Selective abduction will always produce hypotheses that give at least a partial explanation and therefore have a small amount of initial plausibility. In the view concerning abduction as inference to the best explanation advocated by Peirce one might require that the final chosen explanation be the most plausible.

Many attempts have been made to model abduction by developing some formal tools to illustrate its computational properties and the relationships with the different forms of deductive reasoning. Some of the more recent sentential frameworks for abductive reasoning are based on the theory of the epistemic state of an agent, where the epistemic state of an individual is modeled as a consistent set of beliefs that can change by expansion and contraction (belief revision framework). This kind of sentential frameworks exclusively deals with selective abduction (diagnostic reasoning) and relates to the idea of preserving consistency.

Exclusively considering the sentiential view of abduction does not enable us to say much about creative processes in science: it mainly refers to the selective (diagnostic) aspects of reasoning and, when used to express the creativity events it is either empty or replicates the well-known Gestalt model of radical innovation. It is empty because the sentential view stops any attempt to analyze the creative processes. For example, the simple Peircian syllogistic conception of creative abduction - as the fallacy of affirming the consequent - is perfectly compatible with the Gestalt model of discovery. In both cases the event of creating something new (for example a new concept) is considered so radical that incommensurability as regarding the previous knowledge is immediately involved.

I have analyzed elsewhere [3] some limitations of the sentential models in accounting for other reasoning tasks; for example they do not capture 1. the idea of the existence of high-level kinds of creative abductions; 2. the existence of model-based abductions (for instance visual and diagrammatic); 3. the fact that explanations usually are not complete but only furnish partial accounts of the pertinent evidence. If we want to deal with the nomological and most interesting creative aspects of abduction we are first of all compelled to consider the whole field of the growth of scientific knowledge cited above. We may also see belief change from the point of view of conceptual change, considering concepts either cognitively, like mental structures analogous to data structures in computers, or, epistemologically, like abstractions or representations that presuppose questions of justification. Belief revision is able to represent cases of conceptual change such as adding a new instance, adding a new weak rule, adding a new strong rule (see [8]), that is, cases of addition and deletion of beliefs, but fails to take into account cases such as adding a new part-relation, adding a new kind-relation, adding a new concept, collapsing part of a kind-hierarchy, reorganizing hierarchies by branch jumping and tree switching, in which there are reorganizations of concepts or redefinitions of the nature of a hierarchy.

These last two cases are the most evident changes occurring in many kinds of creative reasoning, for example in science. These cases demonstrate the radical incommensurability as regarding previous concepts, that is the cases in which "revolutionary" changes happen and the most "counterinductive" acts can become visible. But the analysis of model-based conceptual change helps us to see the commensurability components of science: different varieties of what is called model-based abduction are related to some of these types of conceptual change. If we want to provide a suitable framework for analyzing the most interesting case of conceptual changes in science we do not have to limit ourselves to the sentential view of theoretical abduction but we have to consider a broader inferential one which encompasses both sentential and what I call model-based sides of creative abduction.

For example we should remember, as Peirce noted, that abduction plays a role even in relatively simple visual phenomena. Visual abduction, a special form of model-based abduction, occurs when hypotheses are instantly derived from a stored series of previous similar experiences. Many visual stimuli are ambiguous, yet people are adept at imposing order on them: "We readily form such hypotheses as that an obscurely seen face belongs to a friend of ours, because we can thereby explain what has been observed".

This kind of image-based hypothesis formation can be considered as a form of visual abduction [4]. We have to say that visual and analogical reasoning are productive in scientific concept formation too; scientific concepts do not pop out of heads, but are elaborated in a problem solving process that involves the application of various procedures: this process is a reasoned process. Visual abduction, but also many kinds of abductions involving analogies, diagrams, thought experimenting, etc., can be just called model-based.

By recognizing the role of these kinds of reasoning the analysis of the reative processes in science is no longer blocked. The analysis of conceptual change can overcome the negative issues that come from the reductionist theory of meaning, and illustrate the various grades of commensurability that can be found when dealing with the roles of model-based abduction in science. My analysis also relates Nersessian’s one [6] exploiting representational and constructive virtues of model-based reasoning and makes use of the Giere’s general idea that "modeling is not at all ancillary to doing science, but central to constructing accounts of the natural world" [1]. I will show how model-based abduction can explain that concept transformation and creation involves the construction of fluid and evolving frameworks that guarantee commensurability at many levels.

The problem of the incommensurability of meaning has distracted the epistemologists from the procedural, extra-sentential and extra-theoretical aspects of scientific practice. Since Kuhn, the problem of translating between languages has dominated the theory of meaning. Manipulative abduction happens when we are thinking through doing and not only, in a pragmatic sense, about doing. It refers to an extra-theoretical behavior that aims at creating communicable accounts of new experiences to integrate them into previously existing systems of experimental and linguistic (theoretical) practices. In manipulative abduction the suggested hypotheses are inherently ambiguous until articulated into configurations of real or imaginated entities (images, models or concrete apparatus and instruments).

In these cases only by experimenting we can discriminate between possibilities: they are articulated behaviorally and concretely by manipulations and then, increasingly, by words and pictures.

Gooding (see [2]) refers to this kind of concrete manipulative reasoning when he illustrates the role in science of the so-called "construals" that embody tacit inferences in procedures that are often apparatus and machine based. The hypothetical character of construals is clear: they are provisional creative organizations of experience: some of them become in their turn hypothetical "interpretations" of experience, that is more theory-oriented, their reference is gradually stabilized in terms of established observational practices. Step by step the new interpretation relates to more theoretical modes of understanding (visual, diagrammatic, symbolic, conceptual), closer to the constructive effects of theoretical abduction. When the reference is stabilized the effects of incommensurability with other stabilized observations can become evident. But it is just the construal of certain phenomena that can be shared by the sustainers of rival theories. Gooding (see [2]) shows how Davy and Faraday could see the same attractive and repulsive actions at work in the phenomena they respectively produced: their discourse and practice as to the role of their construals of phenomena clearly demonstrate they did not inhabit different, incommensurable worlds in some cases.

Finally I will also show how the theory of manipulative abduction can support Thagard’s statement that oxygen and phlogiston proponents could recognize experiments done by each others [8]: the assertion is exhibited as an indispensable requisite for his coherence-based epistemological and computational theory of comparability at the level of intertheoretic relations and for the whole problem of the creative abductive reasoning to the best explanation cited above.

The incommensurability thesis shows some interesting relationships with the moderate and extreme conventionalism: I will illustrate how the theories that are not intertranslatable, that is incommensurable, function in certain respects as do observationally equivalent theories. They are unconcerned by crucial experiments and can be seen as rivals in some sense not imagined in traditional philosophy of science. In these case the role of observational and formal-structural invariants in providing comparability is central: it is impossible to find a contradiction in some area of the conceptual systems they express. I will indicate whether negation as failure can be employed to model the demise of such conventional theories, showing how they can be motivationally abandoned.