_Intuition_ is the faculty of the Mind whereby it evolves what have been called Primary Truths or Primary Ideas. By Primary Ideas are meant the ideas of Space, Time, Cause, Identity, etc. By Primary Truths are meant the so-called "Self-Evident Truths" of geometry, mathematics and logic. Under the head of Intuition are also sometimes included the activities of the Subconscious or Superconscious regions of the mind, of which we have spoken in detail in a volume under that name of this series. Some authorities hold to the older idea of "Innate Ideas" by which is meant that every human being is born with the knowledge of certain fundamental truths, unconnected with any experience. Others hold that these ideas are simply the result of the experience of the race, transmitted to us as "germ ideas" which must grow by experience and exercise. * * * * * That each and every faculty of the Mind may be strengthened and developed by Culture and Exercise is now held to be a fact by nearly every authority worthy of that name. Just as the physical muscle may be cultivated by the proper methods, so may the mental faculties be strengthened and cultivated by the appropriate methods and means. Inasmuch as the majority of the race are deficient in the development of one or more of the leading mental faculties, it becomes a matter of great interest and importance that all should acquaint themselves with the means whereby their deficiencies may be corrected and remedied. We shall now proceed to the consideration of Thought-Culture in general, and then to the consideration of the culture of each particular general faculty, in detail. CHAPTER IV. THOUGHT-CULTURE Thought-Culture is based upon two general scientific facts which may be stated as follows: I. The brain centres of thought may be developed by exercise. While we do not assert that the brain and the mind are identical, it is nevertheless a scientific truth that "the brain is the organ of the mind" and that one of the first requisites for a good mind is a good brain. It has been proven by experiment that the brain-cells concerned in special mental activities multiply in proportion to the active use of the special faculties employed in the mental operation. It has also been ascertained that disuse of special faculties of the mind tends to cause a process akin to atrophy in the brain-cells concerned in the particular activity, so that it becomes difficult to think clearly along those particular lines after a long period of disuse. Moreover, it is known that the education and mental culture of a child is accompanied by an increase and development of the brain-cells connected with the particular fields of thought in which the child is exercised.