8. Each beginner in cookery is therefore urged to master every lesson in the order in which she receives it and to carry out diligently every detail. No lesson should be disregarded as soon as it is understood, for the instruction given in it bears a close relation to the entire subject and should be continually put into practice as progress is made. This thought applies with particular emphasis to the Sections relating to the essentials of cookery. These should be used in connection with all other Sections as books of reference and an aid in calling to mind points that must eventually become a part of a woman's cookery knowledge. By carrying on her studies systematically and following directions carefully, the beginner will find the cooking of foods a simple matter and will take delight in putting into practice the many things that she learns.
9. Each one of the phases of cookery has its importance, but if success is to be achieved in this art, careful attention must be given to the selection of what is to be cooked, so as to determine its value and suitability. To insure the best selection, therefore, the housewife should decide whether the food material she purchases will fit the needs of the persons who are to eat it; whether the amount of labor involved in the preparation will be too great in proportion to the results obtained; whether the loss in preparation, that is, the proportion of refuse to edible matter, will be sufficient to affect the cost materially; what the approximate loss in cooking will be; whether the food will serve to the best advantage after it is cooked; and, finally, whether or not all who are to eat it will like it. The market price also is a factor that cannot be disregarded, for, as has been explained, it is important to keep within the limits of the amount that may be spent and at the same time provide the right kind of nourishment for each member of the family.
10. In order to select food material that will meet the requirements just set forth, three important matters must be considered; namely, the _substances_ of which it is composed; its measure of energy-producing material, or what is called its _food_, or _fuel, value_; and its _digestion_ and _absorption_. Until these are understood, the actual cost of any article of food cannot be properly determined, although its price at all times may be known.