The chapter discusses the intimate relationship between the structure of the human brain and various aspects of thought and mental phenomena. It highlights that all mental processes are explainable through brain structure and activity, and many mental processes occur without brain-cell activity. The complexity of brain structure is essential for mental activities. The text also explores the correlation between general intelligence and brain structure, emphasizing that intelligence is the result of the growth of specific minute structures in the brain. The study of this relationship has been approached by correlating degrees of intelligence with brain structure and by examining the growth of mental abilities alongside brain structure development. The focus is on the nerve cells (neurons) as the physiological units of the nervous system, which are distributed throughout the brain. The human brain, composed of millions of neurons, is divided into three main parts: the forebrain, midbrain, and hindbrain. The forebrain, the largest part, consists of the cerebral hemispheres, while the midbrain acts as a connecting bridge between the forebrain and hindbrain. The hindbrain includes the cerebellum and the medulla oblongata. The chapter includes detailed illustrations of nerve cells and the structure of the cerebral hemispheres, highlighting the importance of the cortex and its "thinking cells" in cognitive functions.The chapter discusses the intimate relationship between the structure of the human brain and various aspects of thought and mental phenomena. It highlights that all mental processes are explainable through brain structure and activity, and many mental processes occur without brain-cell activity. The complexity of brain structure is essential for mental activities. The text also explores the correlation between general intelligence and brain structure, emphasizing that intelligence is the result of the growth of specific minute structures in the brain. The study of this relationship has been approached by correlating degrees of intelligence with brain structure and by examining the growth of mental abilities alongside brain structure development. The focus is on the nerve cells (neurons) as the physiological units of the nervous system, which are distributed throughout the brain. The human brain, composed of millions of neurons, is divided into three main parts: the forebrain, midbrain, and hindbrain. The forebrain, the largest part, consists of the cerebral hemispheres, while the midbrain acts as a connecting bridge between the forebrain and hindbrain. The hindbrain includes the cerebellum and the medulla oblongata. The chapter includes detailed illustrations of nerve cells and the structure of the cerebral hemispheres, highlighting the importance of the cortex and its "thinking cells" in cognitive functions.