Again there are a few terms such as life, death, time and space which cannot be defined satisfactorily. These terms seem to be in a class by themselves or of their own genus (_sui generis_). Since a definition of a term is a brief explanation of it by means of its attributes, it follows that collective terms and terms standing for a single attribute are incapable of definition. Such terms as group, pain, attribute, belong to this class. We may say, then, that there are some terms too high, some too low and some too peculiar to come within the province of logical definition. In short, “_summum genus_,” “_infima species_” and “_sui generis_” are incapable of definition. =9. DEFINITIONS OF COMMON EDUCATIONAL TERMS.= (1) _Development_ is the process whereby the latent possibilities of an individual are unfolded or the invisible conditions of a situation are made apparent. Development means expansion according to principle, while unfolding may or may not involve a principle. (2) _Education_ is the process employed in developing systematically, symmetrically and progressively all of the capabilities of a single life; or (3) _Education_ is the process of modifying experience in order to make the life as valuable as it ought to be. (4) _Teaching_ is the art of occasioning those activities which result in knowledge, power and skill. It is the duty of the true teacher to inspire the child to activity along right lines. Through his own activity the child shapes his inner world which is sometimes termed _character_. Knowledge is anything known, power is ability to act, skill is a readiness of action. (5) _Instruction_ is the art of occasioning those activities which result in knowledge. Instruction develops the understanding; teaching develops character. (6) _Training_ is the occasioning of those activities which, by means of directed exercise, result in power and skill. Training and education are not interchangeable. Training implies an outside authority, while education, which involves inner development, may proceed without supervision. (7) _Knowledge_ is anything acquired by the act of knowing. (8) _Learning_ is the act of acquiring knowledge or skill. (9) Instruction, training, teaching, learning and education all involve activity. Instruction arouses activity which results in knowledge; training directs activity which produces power and skill; teaching includes both instruction and training. Learning is an activity which results in knowledge and skill, while education is a developing process which involves all the others. (10) A _science_ is knowledge classified for the purpose of discovering general truths. (11) _An art_ is a skillful application of knowledge and power to practice. “A science teaches us to know, an art to do.”