Medieval mode of explanation: An action is explained by the object's internal nature which moved it to its final goal or telos.
This mode of explanation was seen to be circular or empty because the only explanation that could be given of an object's essential form or internal nature was in terms of the object's actions. There appeared to be no way to get independent access to a thing's internal nature, as Aristotle had thought reason could do. Hence, the internal nature of the thing was described in terms of its actions. For example, Glass has a brittle internal nature (which means it breaks) or gasoline is inflammable (which means it catches on fire). Since the action was explained by the internal nature and the internal nature was described in terms of the action, the explanation was circular. Saying that glass breaks because it has a brittle internal nature really just says that glass breaks because it breaks.
This caused Galileo to reject the ideas of a secret internal nature as the cause of a things actions. Since it was impossible to perceive and served no explanatory function the idea of an internal nature was rejected. Instead it was seen that only some of the external properties of the object really objectively characterize the object and that these can be used to explain the other external properties.
The properties that were seen as really existing in the object were called primary properties : the properties concerning the mathematical relations of extension and duration. Examples of these are size, duration, shape, movement, penetrability, and number.
The other properties, such as warmth, color, tone, odor, and taste, were called secondary properties. These were not seen as existing in the object, but as ways in which the object affected us, or how it appeared subjectively to us.
A method of hypothesis and experiment under simplified and idealized conditions was devised to isolate out the primary properties and discover the laws that connected them to secondary properties. A concept or model of how the world is (the hypothesis) was tested by deriving predictions about observation from it. But not any observation would serve to test a hypothesis. Our normal observations are a confused morass or tangle of different properties affected by various causes; some of the causes and properties are fundamental and essential to the object or phenomenon, and some of the are merely accidental. The scientist's job is to devise a set of conditions under which she can observe the world so that the real essential properties will shine through and the confusing effects of the accidental properties and causes will be eliminated. She does this by either (1) eliminating the effects of the accidental causes by simplifying or altering the experimental conditions; or (2) Comparing two sets of observations or trials which differ only with respect to the cause or property that is taken by the scientist to be the real cause, the other causes being held constant. This is called the method of controlled experiment: The control being the trial where nothing is changed, and the other trial changing the property or cause that is suspected to be the real cause.
The methodology that arose from the scientific revolution was characterized by:
1. A move towards a Platonic emphasis on the formal mathematical properties that lie beneath the appearances, and away from an Aristotelian reliance on appearances or everyday observations.
2. A tendency to explain all events in terms of the properties of extended matter and their external relationships, i.e., by efficient causality.
3. A method of hypothesis and simplified experiment to discover the laws governing the properties of extended matter.
Along with this methodology came a view of the basic nature of reality. Not surprisingly, nature turns out to have exactly the characteristics that the scientific method is designed to isolate. A simplified version of one of the most prominent versions of this view can be characterized by these theses:
I. The natural world consists entirely of matter extended in space and time. Its properties and relations are exhausted by the properties and relations characteristic of extended matter. This can be further elaborated as follows:
A. Nature consists of qualitatively identical particles: atoms. Each atom is exactly the same as all others with respect to their intrinsic or internal properties. All differences in types of objects result from different ways of combining and arranging these basic elements.
B. The properties of these atoms are exhausted by the formal mathematical properties of extension in time and space: i.e., primary properties. Nature can be completely characterized by properties such as shape, size, number, duration, and speed of change of these other properties. All of these properties are quantifiable and their relations are representable as relationships between numbers.
C. Matter has no internal nature or properties not completely discoverable from an examination of the external properties of extension. Matter has no internal or subjective nature analogous to the internal states of mental substance. It is completely knowable, in principle, from the outside using the objective method.
D. All of the changes and events in nature are completely explainable in terms of the external relations and efficient causal relations of extended matter. All natural causation is either mechanical causation by contact (the billiard ball model) or by forces arising from formal properties of matter (gravitation and action at a distance). There is no final causation or determination by purpose or conceptual content in the natural world. All the relations between objects are external; that is, no change in one atom or object is intrinsically tied to changes in another by internal relationships between their natures. The properties of every atom are independent of all others, except to the extent that they are affected externally by other atoms through efficient causation.
II. The properties of objects that we perceive through the senses (secondary properties) are not really in the objects, but are simply the effects of extended matter on mental substance. (Since primary properties are perceptible by more than one sense and appear differently to different senses, they cannot be said to be perceptible through the senses. They are perceptible only through the mind. They are the real properties of objects.)
This view of Nature is incompatible with religious views of nature and with our common sense view of nature in at least three ways:
1. Life: If nature consists entirely of dead matter, it is difficult to see how life arises from this dead matter. Science solves this by redefining life as a complex type of organization of dead matter.
2. Consciousness: Nothing in nature has an internal nature or inside. Hence, it is difficult to see how humans, have a subjective internal nature, or consciousness, can arise from any combination of matter.
3. Values (and secondary properties in general): All of the tastes, colors, sounds, etc. that we value in the world are secondary properties an really only exist in us not in the world. Likewise, good and bad, pleasure and pain, are only secondary properties and do not really exist in nature objectively.