# Bottom-Up VS Top-Down Method for Finding Laws

Broadly speaking, to discover new regularities and laws we either follow top–down or the bottom–up approach (Fig. 1). In the top–down approach, the search begins with an external observation e.g., Newton’s laws of motion. The observer intuitively imagines a set of elements, a set of interactions and a mathematically expressible form to connect the two. Components are weaved into a mental map and experiments are planned to verify or nullify the model. If the experimental observations repeatedly support the model under different environmental settings, the model takes a more generalized form and may be ultimately adopted, with a broad consensus, as a law.

In the bottom–up approach, one begins by collecting data on individual elements i.e., experimentally determine properties of components in isolation and in association with other interacting elements. Data are collected in different environmental settings and patterns are searched. Once patterns are found, experiments are repeated to confirm observations. The evidence of consistent relationship among interacting components in different environmental settings provides a strong basis to represent patterns in a logical form. This approach was typically used by Gregor F. Mendel to deduce Laws of Inheritance (Mendel 1865).

In both the approaches, scientists contribute their own subjective judgment in terms of what is contingent (exceptions to the rule) and what is essential (obeying the rule). The extent of exceptions and commonalities vary among different instances and clearly has to do with the scale at which the observations are made.

In both top–down and bottom–up approaches, the key is to find a consistent pattern. For example, an equation consistently explaining regularity is a strong indication of a law. The top–down approach i.e., from imagination to observation, has been often used in physics, while the bottom–up approach i.e., from observation to imagination has been used in biology. Interestingly, we have laws for things that we cannot see e.g., light, gravity and sound, but no laws for things that we see e.g., DNA, RNA, proteins and cells. This is due to the fact that former are based on the consistent behavior of elementary particles compared to the latter where interactions are frequently probabilistic.

Going further, one understands that the well-known law of gravity is nothing but a name given to the striking regularity observed in the motion of the bodies. However, even this regularity is obtained by a subjective choice of what is essential. Pure observation tells us that some bodies e.g., leaves on a windy day, go up and down and not directly down towards the earth. Due to this reason Aristotle spoke about two kinds of bodies: light and heavy. Only in the XVII century Galileo decided to think of the difference between lightness and heaviness as contingent and identified the tendency to fall down (gravity) as the key feature. Thus the concept of gravity is essentially a rationalization of the observed behavior of bodies. The search for the material counterpart of this force in terms of particles (gravitons) is still elusive and highly uncertain. In the same way if we clap our hands a nearby mouse will surely run away with a reliability degree of predictability, comparable to that of falling bodies. However, if we try to explain this very repeatable pattern in terms of mouse microarray profile, before and after the clap we will surely have a hard time. The key message is that the molecular level description is sometimes inadequate to explain higher-level behavior of organisms.

## Notes:

Fascinating explanation for why laws are harder and harder to find as we move into macroscopic sciences.

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Laws of biology: why so few?
Periodicals>Journal Article:  Dhar, Pawan K. and Giuliani, Alessandro (March, 2010), Laws of biology: why so few?, Syst Synth Biol, Retrieved on 2014-04-30
• Source Material [www.ncbi.nlm.nih.gov]
• Folksonomies: theory law