The work of the Duke of Argyll offers an in-depth exploration of philosophical themes, particularly the unity of nature and the principle of lawfulness. In this fifth edition, the relevance of the author's arguments is highlighted, distinguishing them from the views of contemporary thinkers. The author expresses gratitude for the critical feedback provided in a respectful tone and has made no fundamental changes to his principles in this edition. Instead, some passages have been revised to clarify misunderstandings, and substantial additions have been made to address the arguments of his opponents. Discussions that did not fit into the main text are summarized in notes at the end of the book. This book is a valuable resource for anyone interested in the philosophical reflections of the Duke of Argyll.