The role of instinct has been justifiably neglected by the mainstream readers of Charles Peirce's philosophy. The aim of this project is to examine Peirce's theory of instinct in terms of his distinction between theory and practice. Peirce introduced a clear distinction between theory and practice and also attempted to link them together. Instinct has a double-edged role in linking theory and practice. In practice, instinctive beliefs guide our acts as norms to which we conform. In theory, instinctive reason passes through the course of reasoning by providing simple hypotheses. "A Neglected Argument" (1908) is the very example of Peirce's attempt to link theory and practice. We have instinctive beliefs in the idea of God and we are infinitely inspired into inquiry by it. Musement leads one to contemplate the idea of God, and this contemplation in turn leads one to a belief in God's reality. The idea of God is instinctively apprehended by human minds in the form of judgments and our doubting the judgments is the starting point of scientific inquiry. In this sense, the human inquiry begins with instinctively and commonsensically given judgments without exception.