Purposeful Design
The Explanation
Purposeful design is proposed as an explanation for biological organization when characteristics of living systems are thought to resemble effects that, in other settings, are known to result from intelligent activity.
The argument is not simply that living systems are complicated. Intelligent Design proponents instead point to particular forms of organization, including functionally specified information, coordinated molecular machinery, integrated regulation, error correction, feedback control, and systems in which multiple components must cooperate to perform a function.
Meyer argues that functionally specified biological information is relevant because intelligent agents are known to produce information-rich systems. Behe approaches the question from another direction, arguing that some biochemical systems contain multiple interacting components whose coordinated function presents a challenge to gradual evolutionary explanations.
From this perspective, purposeful design is proposed as a causal explanation to be compared with natural selection, self-organization, historical modification, constraint, and other proposed explanations rather than merely being inferred from the fact that biological systems are complex.
Supporting Evidence
Living systems contain genuine forms of organization that invite investigation. DNA stores sequence-dependent biological information. Molecular systems coordinate the expression and use of that information. Cells employ feedback, signal processing, error correction, robustness, modular organization, and interacting regulatory networks.
Meyer argues that intelligent agents provide an independently known example of a cause capable of producing functionally specified information. He therefore proposes intelligent activity as an explanation for the origin of biological information rather than treating design merely as an analogy between organisms and machines.
Behe emphasizes integrated biochemical systems. His argument focuses upon cases in which several interacting components contribute to a system-level function and asks whether plausible evolutionary pathways can account for the origin of that coordinated organization.
Taken together, these arguments propose that biological information and integrated functional organization may contain characteristics that warrant consideration of purposeful causation.
Limitations
The presence of complexity, function, or engineering-like organization does not by itself establish intelligent causation. Earlier parts of this investigation have shown that natural selection, self-organization, inherited regulatory architecture, historical contingency, constraint, and convergent solutions to functional problems can all contribute to biological organization.
Critics of Intelligent Design argue that evolutionary processes can generate increasing complexity through modification, duplication, interaction among molecular components, selection, and other mechanisms. They also dispute the claim that present dependence among the components of a system demonstrates that the system could not have arisen through earlier stages having different functions or arrangements.
Design arguments therefore need to examine actual biological histories and proposed mechanisms rather than infer design merely because the present system is highly integrated or resembles human engineering.
Questions That Remain
The central question is whether particular biological systems contain features for which purposeful organization provides a better explanation than the available natural mechanisms and historical pathways.
This requires detailed investigation. How much functionally specified information can known processes generate? What intermediate stages are available for highly integrated systems? Can components acquire new functions or be recruited from earlier systems? How much organization can arise through selection, self-organization, constraint, and network evolution?
There is also the broader question of what evidence would distinguish purposeful organization from biological structures that merely resemble designed systems because similar functional problems favor similar solutions.
Intelligent Design Response
Intelligent Design proposes that purposeful causation should remain among the explanations considered when biological systems display information-rich and highly coordinated functional organization.
Its positive argument is that intelligent activity is a known cause of functionally specified information and purposeful arrangements of interacting components. Proponents therefore ask whether comparable characteristics in living systems may reasonably point to a similar kind of cause.
This inference remains disputed. Evolutionary biologists argue that mutation, selection, duplication, co-option, self-organization, historical inheritance, and other processes can produce complex biological organization without requiring intelligent intervention.
The investigation therefore does not end merely by observing that life resembles engineering, nor merely by identifying evolutionary mechanisms that can produce biological change. The question is whether the detailed evidence from particular systems is better accounted for by the natural processes and historical pathways that can be demonstrated, by purposeful organization, or by some combination or explanation not yet fully understood.