Archive for January 10th, 2011

January 10, 2011

Purpose (2) – Teleological explanation

by Neil Rickert

Briefly, a teleological explanation is an explanation based on a purpose.  The thermostat, for example, has the purpose of maintaining a suitable temperature, and we often explain its operation in terms of how it meets that purpose.  Biologists sometimes talk of teleonomic explanations.  An explanation is said to be teleonomic if it is based on apparently purposeful behavior, and teleological if it is based on a purpose arising from a conscious agent.  Since I am not particularly concerned with the role of consciousness here, I shall not make that distinction and will use “teleology” to describe both cases.

Mechanistic explanation

Teleological explanation can be contrasted with mechanical explanation.  A mechanical explanation is one based on the idea of physical matter in motion, as described by laws of physics.  When we describe the behavior of objects using Newton’s laws of motion and Newton’s law of gravity, we are providing a mechanistic explanation.  No purpose is assumed by such an explanation, so the mechanistic explanation is entirely non-teleological.  Scientists usually prefer mechanistic explanations, where they are available.

Chaotic behavior

Mechanistic explanation can break down, when there is chaotic behavior.  Chaos, as I am using the term, is well described by:

Mathematically, chaos refers to a very specific kind of unpredictability: deterministic behaviour that is very sensitive to its initial conditions. In other words, infinitesimal variations in initial conditions for a chaotic dynamic system lead to large variations in behaviour.

When a mechanistic explanation deals with chaotic behavior, the explanation is of limited use.  In particular, it is unable to make reliable predictions.

Example – the thermostat

We can think of the thermostat typically used as part of a heating system for a house.  The thermostatically controlled system keeps the house at a near uniform temperature.  The thermostat, as a simple device, is part of the controlling functionality.  If we ignore the electrical aspects, then a thermostat has a simple mechanistic description.  In the traditional version, a bimetallic strip bends as it is heated, due to the differential expansion of the two metals.  And this bending brings two surfaces (usually part of a switch) into contact.  Once we include the electrical characteristics, things become a bit more complicated.  The switch does not instantly go from open to closed.  Rather, the resistance of the circuit goes from very high (open circuit) to very low (closed circuit).  The transition in resistance is chaotic, and that limits the accuracy of a mechanistic account of what happens.  However, the fact that the electrical transition is chaotic does not interfere with the intended purpose of the thermostat.  So we can give a good teleological account without getting into the details of the chaotic behavior.

Pseudo-mechanistic explanation

Typically, the full thermostatically controlled system is explained by describing the thermostat as switching from open to closed (or from off to on) when it reaches the set temperature.  But when we describe it that way, we are not giving a mechanistic account of the thermostat, for we are not talking about parts in motion.  We have, in effect, replaced the thermostat in our description with an abstract ideal machine that just switches.  The full explanation of the thermostatically controlled system, when given that way, has the general form of a mechanistic explanation, except for our substitution of the ideal abstract machine for the actual mechanism of the thermostat.  I shall use the term “pseudo-mechanistic” for an account that has the general form of a mechanistic explanation, but is based on the “mechanism” of an abstract ideal machine.  Such an explanation is implicitly teleological, for we have constructed that abstract ideal machine based on the intended purpose of the actual thermostat.

When it comes to pseudo-mechanistic explanation, the big example is the digital computer.  We typically explain its operation in terms of logic gates, flip-flops, latches, etc.  The flip-flop is used as a one-bit memory device.  Electrically, it is a transistor like component, but with the transistors operating in a non-linear region.  We normally describe it as having two stable electrical states, and being switchable between the two.  The states might be indicated by a voltage or a current flow, depending on chip design.  The transition from one electrical state to the other is chaotic.  We often describe this as a memory cell which can have the value 0 or 1, and in using that description we do not mention the electrical values.  Likewise, a logic gate is usually described as having an output value of 0 or 1, depending on the inputs.  Again, the actual physical device has output voltages or currents (depending on chip design), and the transition between the output levels that we label “0” and “1” is chaotic.  In typical computer explanations, we describe the logic gate as an idealized abstract machine that can have an output of 0 or 1.  Our explanation of computer operations is in the form of a mechanistic explanation, except that it is based on these abstract ideal machines such as logic gates.  And our use of abstract ideal machines is based on the intended purpose of the actual electrical circuits, so is implicitly teleological.  There’s a bit of an irony here, for AI (artificial intelligence) proponents are often outspoken in their favoring a mechanistic view of everything, yet they rely on a teleological account of their computers.

Purpose in biology

When the inputs to a neuron reach a sufficient level, the neuron “fires” and transmits a signal.  This is usually described as a threshold event, with the neuron activating (or firing) when the input reaches a threshold.  With threshold events, there is a large output change from a small input change (that last little bit of input that pushed to the threshold).  And because of that, we should consider the operation of the neuron to be chaotic.  This is an example of an apparently purposeful action of a biological cell, though it is hard to be precise about what we should consider the purpose, since the operations of the brain are not yet fully understood.

When we talk of a struggle for survival, we are using teleological language, and assuming some sort of intrinsic survival purpose in the biological organism.  If we say that the purpose of flowers is to decorate our living rooms or our gardens, then we are imposing our own purposes on the plants.  If, however, we say that the flowers have the purpose of increasing the likelihood of successful pollination, then we are ascribing a purpose which is a better fit to what the plant appears to be doing.  It is difficult to discuss biology without the use of teleological language, because the appearance of purposeful action is so common.


I have illustrated how widespread is our use of teleological language.  At the same time, I have suggested we often run into situations where a purely mechanistic explanation is unsatisfactory, often because there are chaotic aspects to the behavior we are describing.