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mechanismandcircadianphenomena - Mechanisms and Delineating...

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11/15/11 1 Mechanisms and Delineating Circadian Phenomena ...a rose is not necessarily and unqualifiedly a rose it is a very different biochemical system at noon and at midnight. — Colin Pittendrigh, 1965. Nomological vs. Mechanistic Explanations Under the influence of paradigmatic examples in physics, in the 1950s Carl Hempel advanced an account of explanation in terms of laws Laws (e.g., Boyle-Charles gas law) Initial conditions Description of phenomenon to be explained (e.g., increase in temperature of gas) Laws: true universal generalizations with counter-factual import Specify what would happen if initial conditions were satisfied Problem: there aren’t many examples of such laws in biology, but lots of explanations These offer accounts of mechanisms claimed to produce the phenomenon Conceptions of Mechanism Machamer, Darden, and Craver [MDC] (2000) – “Mechanisms are entities and activities organized such that they are productive of regular changes from start or set-up to finish or termination conditions” Bechtel and Abrahamsen (2005, cf. Bechtel and Richardson, 1993): – “A mechanism is a structure performing a function in virtue of its component parts, component operations, and their organization. The orchestrated functioning of the mechanism is responsible for one or more phenomena” Besides the incidental differences in vocabulary, the major difference involves the last phrase of MDC— imposing an order from start to termination conditions
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11/15/11 2 Features of Mechanistic Explanations Dualism of Entities and Activities – Activities (operations) are the producers of changes Types of causings – Entities (parts) are the things that engage in activities Organization: “Entities often must be appropriately located, structured, and oriented, and the activities in which they engage must have a temporal order, rate, and duration” – Productive continuity: operations must link entities into a continuous network Contrasts with Nomological Account Both nomological and mechanistic explanations can be concerned with causal phenomena—something happens which brings about something else Nomological explanations focus on the regularity in the change itself Critical feature of mechanistic accounts is that they focus on the system in which change is occurring and ask what is going on inside to produce its behavior Nomological explanations emphasize linguistic representations and logic Logic is the glue that relates laws to actual cases Mechanistic explanations focus on the entities and activities What are the operations performed that together bring about the effect? Mentally Imagining Mechanisms An early example in Machamer, Darden, and Craver: – In the mechanism of chemical neurotransmission, a presynaptic neuron transmits a signal to a post- synaptic neuron by releasing neurotransmitter molecules that diffuse across the synaptic cleft, bind to receptors, and so depolarize the post-synaptic cell
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