What are referred to as the secrets of nature and principles of intellectual truth?

The "secrets of nature and the principles of intellectual truth" are the peculiar objects of study and discovery associated specifically with the Second Degree, that of a Fellowcraft. During this stage of instruction, a candidate is led to contemplate the human intellectual faculty and trace its development through the paths of heavenly science, as an article on The Square Magazine's website notes. This intellectual and scientific focus is later explicitly recalled in the retrospective address delivered in the Third Degree, which summarizes the candidate's progression through the Craft degrees.