p Natural sciences command a vast array of facts showing that living nature arose out of non-living, inorganic nature. There is no impassable boundary between them. Chemical analysis shows that both inorganic bodies and living organisms are formed from the same chemical elements. Organisms contain large quantities of hydrogen, oxygen, nitrogen and particularly carbon, which comprises the basis of the chemical composition of living organisms and the products of their vital processes.

p Scientists have put forward the hypothesis that the primary gas-dust matter from which our Earth was formed originally contained the simplest compounds of carbon and hydrogen, hydrocarbons, from which the more complex organic compounds were subsequently formed. Entering into chemical associations with each other, the organic compounds became more and more complex until amino acids, the basic elements of the protein molecules, were formed. As organic substances became more differentiated and complex, their reflective ability became more diverse and intricate.

p Hundreds of millions of years later the molecules of this primary chemical protein, formed from amino acids, had turned into a living protein body and thereby acquired the property of metabolism, which is the basic feature of everything living. Landing in a favourable environment and entering into a metabolic interchange with it, this protein body became an organism.

p Metabolism is a contradictory process of assimilation (absorption of nutritive substances from the environment and their conversion into the living cells and tissues of an organism) and dissimilation (disintegration, destruction of this living tissue). This process is inherent only in living protein, in an organism. Metabolic exchange with the environment and constant self-regeneration differentiate the simplest living organism from the most complex non-living body. Only by assimilating nutritive substances and excreting the products of their disintegration, can an organism live and develop. “Life is the mode of existence of protein bodies, the essential element of which consists in continual metabolic exchange with the natural environment outside them, and which ceases with the cessation of this metabolism,”  [64•*  Engels points out.

p The coming into being of the first simplest organisms was a tremendous step forward in the development of reflection, a general intrinsic property of matter, in the emergence of consciousness. Reflection of reality, inherent in inorganic nature, turned into a qualitatively new, biological reflection. The simplest form of biological reflection is response to stimuli, irritability, which is inherent in all organisms and serves as a means of their orientation, or adaptability to the external environment.

p Plants, for example, are especially sensitive to sunlight. They literally reach out for it; for them light is the source of life. The simplest monocellular organism, the amoeba, reacts to food stimuli, but if it has just swallowed food, the food stimuli have no effect upon it. This means that the amoeba, like any other organism possessing the property of responding to stimuli, reflects the outside world not passively, but selectively. Its organism, as it were, gravitates towards useful, needed stimuli and shuns harmful, unnecessary ones. 65 But its selective power is not great. A simple organism has neither organs nor tissues, nor cells, specially receptive to particular forms of stimuli. It responds to outside excitations in its entirety.

p In the course of further evolution, as the organisms themselves and the environment became more complex, an even higher form of reflection, sensation, arose on the basis of response to stimuli. Like response to stimuli, sensation was a result of the action of the outside world on the organism, but here the range of external stimuli to which the organism responded in one way or another broadened considerably. The organism reacted to colour, smell and sound, it developed the sensations of taste, cold, heat, moisture and responded to mechanical, physical and other influences. Organs capable of perceiving only a definite range of external influences (colour, sound, smell, etc.) appeared in the organism. Subsequently, as the organisms developed, their sensations became more subtle and diverse. The adaptability of the organism to the environment increased and a special organ for maintaining contact with the environment, the central nervous system, came into being.

p In the field of biology the study of reflexes has graphically shown that lower and higher animals do not have the same ability to reflect the surrounding world, to adapt to the environment. Reflexes are responsive reactions of the organism to external influences, as well as to its own internal changes. All of them are divided into unconditioned and conditioned reflexes. Unconditioned reflexes are inherent in all organisms, both lower and higher, and are inborn, hereditary. A man instantly draws away his hand if it is touched by something hot—this is an unconditioned reflex. The intricate intertwining of unconditioned reflexes forms instincts (sex, food and others), which play a major role in the life and development of an organism.

Higher animals, however, also have conditioned reflexes which are of a temporary nature and are formed in definite conditions. If for a certain time a dog is fed to the accompaniment of the ringing of a bell, there will come a moment when the dog reacts to the ringing of a bell in the same way as when it is fed: saliva will be secreted. A temporary connection has been formed in the brain of the dog whereby the sound of the bell has become the signal for food. All 66 other conditioned reflexes are formed on the same principle. Thanks to them the organism adapts itself very delicately to the environment and is very sensitive to its influences. The conditioned reflexes which acquire particular importance for the organism become fixed and turn into unconditioned ones; on the basis of the latter new temporary connections arise, part of which again become fixed. Hence, in the course of the evolution of living organisms psyche progressed continuously, and this ultimately led to sentient matter acquiring the ability to think.