Nancy Cartwright deals with the question of natural laws and their relation to reality. Her first book is named "How the laws of physics lie". Sometimes this book was understood as a attack against realism. In her later books we can see that her aim is only to destroy fundamentalism.

Her ontological starting point is that the world is really dappled and not submitted under rigid structures of order and lawfulness (Cartwright 1999). Laws are not all embracing structures of order, how they are often described in deductive-nomological models of explanation. Laws shows themselves only under certain conditions and they are based on capacities, in the "nature of things", which are not stable occurent properties, but which act in dependence of certain conditions. On the road from reality to laws there is needed a so called nomological machine. The nomological machine produces certain conditions in order to prepare the main factors in a determined way and to create the lawful behaviour.

Cartwright assumes that science is not representable by a unifying pyramid of theories, but by a patchwork.

We are compelled to use models which determine and therefore restrict the validity of the theory. "So the laws of physics apply only where its models fit, and that, apparently, includes only a very limited range of circumstances." (Cartwright 1999: 4)

In her first book "How the laws of physics lie" (1983) Cartwright criticised the view laws could be true or false in an immediate relation to reality.

It is necessary to remodel the appearances of reality in descriptions and models and to match them to the mathematical representation of the theory. Cartwright develops a model of "simulacrum-explanation". The explanation of a phenomenon requires the construction of a model. The law holds for the objects of the model, not the phenomena themselves. The objects carry only the form or the appearances of the real thing, not the substance and qualities. Therefore they are only simulacrum.

According to Cartwright science can’t directly understand the order of the world. The "dappled" world shows an interacting change of many causal relations; its relation to order and laws is not immediately. We need an arrangement which filters or produces that order we know as laws. This arrangement is called "nomological machine" by Cartwright. The nomological machine is "a fixed (enough) arrangement of components, or factors, with stable (enough) capacities that in the right sort of stable (enough) environment will, with repeated operation, give rise to the kind of regular behaviour that we represent in our scientific laws" (Cartwright 1998a:2). Such a nomological machine exists in nature very seldom. The solar system is such a machine. Otherwise we have to built such a machine in our scientific work. To a nomological machine belongs the concrete material devices and the ideal models and concepts..

The nomological machine is a philosophical concept (Cartwright 1998a: 10), a "a way of categorizing and understanding what happens in the world". In this sense Cartwright alters empirism: There are presuppositions, which can’t be tested within the frame of the theory itself.

According to Cartwright laws are "descriptions of what regularly happens, not regular associations or singular causings that occur with regularity" (Cartwright 1999: 4). Laws are not necessary associations between measuring quantities, laws need including the conditions. These conditions are given through the nomological machine. Then we can reconstruct laws.

But the nomological machine is not only a construction that creates laws. There is something in nature, what gives the base to lawfulness behaviour disregarding the dappled nature of the world. These are the natural capacities, the nature of the things to carry properties under certain conditions.

We can’t examine in our scientific work which cause causes which effect. But we can find out, how the things "try to do something" (Cartwright 1999: 28). To what extend the things are successful in their trying, depends on circumstances. When we know the laws, we don’t know "what happens" or "occurent properties", but "what is in their nature, to do" (ibid.: 82) or about their capacity, to have properties (Cartwright 1998b: 88).

Cartwright specified her criticism about realism by explaining that she assumes that laws don’t have a real existence, but that "capacities are real" (Cartwright 1989/1994: 1).

Capacities are not the same as powers or dispositions. "Disposition-terms [...] are tied one-to-one to law-like regularities, and philosophers are often at debate about which is more primary. But capacities […] are not restricted to any single kind of manifestation. Objects with a given capacity can behave very differently in different circumstances." (Cartwright 1997: 74, see also Cartwright 1998a: 12)

A theory which uses such capacities instead of fixed and all embracing laws is quite more open (Cartwright 1998a: 12).

For example the Second Newtonian law doesn’t deal with "occurent properties" like masses or distances, but with forces and that means: the capacity of one body to move another body. Therefore under different conditions may emerge different laws (the Keplerian law of planets or the Galilean Law about the fall of a physical body). Another example:

If we take into consideration capacities we can assume that scientific knowledge may be true without fundamentalist pretension. Laws express the effectiveness of the real existing natural capacities under certain conditions. What really happens can’t directly be deduced from laws (as assumed in the deductive-nomological model of explanations). Real events are not directly determined by laws; they base on complex interactions. Therefore the "patchwork pluralism" of Cartwright doesn’t fight against realism but against intellectual imperialism. (Cartwright 1997/2002: 209)

By using capacities as basis of laws the "law-first view" (Cartwright 1997: 69) is criticised.. She emphasises that she combats against unconditional and unlimited use of laws (Cartwright 1998a:: 12). Understanding the world is not a task of recognising laws, but of grasping capacities. The point of view changes: It is not said that something leads necessarily to another thing, but we say that something allows other things or another behavior (see Cartwright 1998a: 27). Using a nomological machine we can reach a chosen goal. The object of science are not longer things and their given properties but "what a property empowers an object to do" (Cartwright 1998a, S. 25).

Ian Hacking distinguished between representing and intervening as task of science. The "representing people" ask "How can the world be the way science says it is or represents it to be?" But Cartwright asks: "How can the world be changed by science to make it the way it should be?" (Cartwright 1999: 5).

Cartwright, Nancy (1983): How the laws of physics lie. Oxford, New York: Clarendon Press.

Cartwright, Nancy (1989/1994): Nature’s Capacities and their Measurement. Oxford: Clarendon Press.

Cartwright, Nancy (1998a): Where Laws of Nature Come From? In: Paul, Matthias (Hrsg.): Nancy Cartwright: Laws, Capacities and Science. Vortrag und Kolloquium in Münster 1998. Münster: LIT-Verlag 1998. S. 1-30.

Cartwright, Nancy (1998b):Comments and Replies. In: Paul, Matthias (Hrsg.): Nancy Cartwright: Laws, Capacities and Science. Vortrag und Kolloquium in Münster 1998. Münster: LIT-Verlag 1998. S. 88-109.

Cartwright, Nancy (1997): Where Do Laws of Nature Come From? In: Dialectica, Vol. 51, No.1 (1997). S. 65-78.

Cartwright, Nancy (1997/2002): Warum Physik? In: Penrose, Roger (1997/2002): Das Große, das Kleine und der menschliche Geist. Heidelberg, Berlin: Spektrum Akademischer Verlag. 2002. S. 201-210.

Cartwright, Nancy (1999): The Dappled World. A Study of the Boundaries of Science. Cambridge: Cambridge University Press.