The book is so big that I must make no longer preface, than to give you this necessary, short account, I. Of the quality; II. And the reasons of this work.
1. The matter you will see in the contents: As Amesius's "Cases of Conscience" are to his "Medulla," the second and practical part of theology, so is this to a "Methodus Theologiæ" which I have not yet published. And, 1. As to the method of this, it is partly natural, but principally moral; that is, partly suitable to the real order of the matter, but chiefly of usefulness, _secundum ordinem intentionis_, where our reasons of each location are fetched from the end. Therefore unless I might be tedious in opening my reasons _à fine_ for the order of every particular, I know not how to give you full satisfaction. But in this practical part I am the less solicitous about the accurateness of method, because it more belongeth to the former part, (the theory,) where I do it as well as I am able.
2. This book was written in 1664 and 1665 (except the Ecclesiastic Cases of Conscience, and a few sheets since added). And since the writing of it, some invitations drew me to publish my "Reasons of the Christian Religion," my "Life of Faith," and "Directions for Weak Christians;" by which the work of the two first chapters here is more fully done; and therefore I was inclined here to leave them out; but for the use of such families as may have this without the other, I forbore to dismember it.
3. But there is a great disproportion between the several parts of the book. 1. The First Part is largest, because I thought that the heart must be kept with greatest diligence, and that if the tree be good the fruit will be good; and I remember Paul's counsel, "Take heed unto thyself, and unto the doctrine; continue in them: for in doing this thou shalt both save thyself, and them that hear thee," 1 Tim. iv. 16. Nothing is well done by him that beginneth not at home: as the man is, so is his strength, and work. 2. The two first chapters are too coarse and tedious for those of the higher form, who may pass them over. But the rest must be spoken to; to whom that is unprofitable which is most suitable and pleasant to more exercised and accurate wits. The grand directions are but the explications of the essentials of christianity, or of the baptismal covenant, even of our relation-duties to God the Father, Son, (in several parts of his relation,) and of the Holy Ghost. The doctrine of Temptations is handled with brevity, because they are so numerous; lest a due amplification should have swelled the book too much; when a small part of their number maketh up so much of Mr. John Downame's great and excellent treatise, called, "The Christian Warfare." The great radical sins are handled more largely than seemeth proportionable to the rest, because all die when they are dead. And I am large about Redeeming Time, because therein the sum of a holy, obedient life is included.
4. If any say, Why call you that a Sum of Practical Theology which is but the directing part, and leaveth out the explication, reasons, various uses, marks, motives, &c.? I answer, 1. Had I intended sermonwise to say all that might well be said on each subject, it would have made many volumes as big as this. 2. Where I thought them needful, the explication of each duty and sin is added, with marks, contraries, counterfeits, motives, &c. And uses are easily added by an ordinary reader, without my naming them.
5. I do especially desire you to observe, that the resolving of practical cases of conscience, and the reducing of theoretical knowledge into serious christian practice, and promoting a skilful facility in the faithful exercise of universal obedience, and holiness of heart and life, is the great work of this treatise; and that where I thought it needful, the cases are reduced to express questions and answers. But had I done so by all, many such volumes would have been too little; and therefore I thought the directing way most brief and fit for christian practice; for if you mark them, you will find few directions in the book, which may not pass for the answer of an implied question or case of conscience; and when I have given you the answer in a direction, an ingenious reader can tell what question it is that is answered. And so, many hundred cases are here resolved, especially in the two first parts, which are not interrogatively named.