Table 1.

Elaboration on the fundamental concepts.

Concept	Definition
Matter-energy	In his theory of living systems, Miller (1995) refers to matter and energy jointly in order to follow the principle of mass-energy equivalence established in physics. This principle may seem remote from the problems of biology. Nonetheless, the joint term of matter-energy is valid for living systems, because for biological organisms matter and energy are biochemically inseparable. Interconversions of energy during chemical reactions alter the underlying chemicals (particles of matter), and are fundamental to the process of life. The term is therefore useful for capturing the total flux of enthalpy through an organism
Information	Matter-energy and information are related. Information is always borne on a material marker. Distribution of matter-energy in the environment is inhomogeneous, and thus embeds information. An organism’s material components interact with each other, communicating and changing their states, which is a form of information flow. Thus, in organisms the fluxes of matter-energy and information are like two sides of a coin: somewhat different, but not entirely separable.
	Information arises from the environmental inhomogeneity. Thus, the spatial and temporal variability in ecological niches are literally measured by the amount on information to which the organism is exposed. Because these concepts are very abstract, we choose to express the organism’s environment in terms of signals
Signal	This term is used to describe what is happening in the external environment as well as within an individual organism. Depending on the context, signals may describe physical events and quantities, values of physical quantities, as well as patterns formed by values of physical quantities. Examples of signals include chemical: nutrients, pH, salinity, moisture, etc.; physical: temperature, pressure, illumination, etc.; social: proximity to and signals issued by other organisms, etc.
Flexibility mechanisms	These mechanisms are expressed either through inner changes or through outward behaviors. Examples include: inner changes: gene expression patterns, intracellular signaling cascades, heart rate modulation, stomach juice secretion, melatonin production, subcutaneous fat accumulation and loss, learning, etc.; outward behaviors: movement of any kind, taxis, ingestion, egestion, pheromone secretion, leaf/tail shedding, etc. An organism is considered more flexible if it is able to respond with a greater number of inner changes or outward behaviors to a greater number of informative patterns in its environment. The latter can be represented by a greater number of different physical quantities, or by a broader range of values of the same physical quantity, or by greater complexity of spatio-temporal patterns formed by the values of a physical quantity. In the text, all of these representations are referred to as “signals”
Robustness properties	Features and properties that make an organism less vulnerable with respect to signals it cannot process. Examples include: cell wall, bark and thorns, skin, fur, horns, teeth, shells, claws and skeleton, constitutively produced poison or other chemical repellant, thermophilic proteins, etc. An organism is considered more robust if it is able to withstand without change a greater number of informative patterns (signals) in its environment