In this paper, I have tried to present, by a descriptive – analytic method, an analytical description of, the eighteenth century Scottish philosopher, Thomas Reid's ethical theory - which is known as "Common-Sense Intuitionism" – on the way of perceiving and acknowledging the moral values and norms, and through this analytical description, answering these two questions: "How far does the scope of Reid's ethical intuitionism contain?" and "What is Reid's explanation of the origin of moral error?" With reference to the (...) above-mentioned analytical description, and by clarifying some of the pros and cons of Reid's ethical theory, I have tried to show, first, "Whether Reid's ethical theory is a justified, defensible and acceptable theory in justifying the validity of ethical knowledge?" And then, “Whether Reid's explanation of the origin of men's moral error is a sufficient explanation?". (shrink)