Causal Link and Causal Law
It remains to be shown how we can certify that causal links obtain or, eventually, that some causal laws are true. This is an important task. The justification of our assertions of existence hinges on the reliability of the supposed causal connections between what we observe and what exists.
When perception is immediate, no causal link need to be invoked. The actual presence of a glass of beer immediately evidences its existence beyond reasonable doubt in a normal context of perception. Direct or immediate realism is the indispensable ground for epistemological realism about external things.By reflecting on our perceptions, we are in position to ascertain the invariance as well as the orderly variation of some properties of the perceived thing, e.g. when our spatial points of view are modified. If we alternatively shut and open our eyes, we perceive the same properties, provided the perceptual environment remains unchanged. These banal observations provide sound reasons to believe that some causal connections are in place between the perceived concrete objects and the perceptions of their properties.
To discover and mathematically formulate causal laws, thorough empirical investigations and in-depth theoretical work are necessary. As a first stab, we can fall back on the well-known methods of agreement—and above all of difference— of John S. Mill (1911). Think of Newtonian dynamics. If we have reasons to believe that forces exist, it is in the first place because of our bodily experience. Weighty things exert a force on our arms when we carry them. An impressed force also produces variations of velocity. A body at rest can be put in motion by exerting pressure on it. When no net force acts, there is no acceleration. In science we must denote a force by a mathematical symbol, namely a vector. Moreover, we must be able to measure forces with accuracy. In order to measure the gravitational force, for example, we can use a scale.
When it is horizontally in equilibrium, we can conclude that the forces, called “weights”, exerted on the arms of the scale are equal. This static way to measure certain forces can be generalized to measure the values of different forces in some unit.Accelerations can be measured by means of various experimental devices, such as the Atwood machine. In numerous cases, we can measure the forces and the accelerations they cause in order to verify that accelerations are proportional to forces, in conformity with the fundamental law of Newtonian mechanics. The proportionality factor—the inertial mass—can be measured by the method of Huygens' collisions, independently of Newton's second law. When the vectorial sum of the forces is (approximately) nil, the velocity of the mobile is (approximately) constant in magnitude and direction. This is the inertial motion. Numerous observations and measurements give excellent reasons to believe that the law of inertia holds for all bodies.
Perrin's argumentation for the existence of the molecules can then rely on the fundamental laws of mechanics, which can furthermore offer a causal explanation of the property of gas pressure. We must therefore distinguish several levels in a causal explanation. The first hinges on the mechanisms described by a theory when Perrin relies on the kinetic theory and the laws of collision to explain what is going on in an emulsion. The second level consists in the causal interactions between our apparatuses, such as a microscope, and some observed properties, like the distribution of grains in an emulsion. Ii is not necessary to know the laws which govern the interactions of the second level to be entitled to believe that some objects having not directly observable (OP) properties exist.
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