about 145 AD, which demonstrates that Ptolemy wrote the Tetrabiblos towards the end of his life, sometime between completing the Almagest and his death, generally reported to be around 168 A.D. Book I: principles and techniques Opening chapters The text begins with Ptolemy's address to "Syrus", an unidentified character to whom all Ptolemy's works are dedicated. In this Ptolemy differentiates between two types of astronomical study: the first (astronomy proper) which discovers the astronomical cycles and movements; the second (astrology) which investigates the changes these movements bring about. He states that each has its own science and the former is desirable in its own right "even though it does not attain the result given by its combination with the second". This is taken to demonstrate Ptolemy's view that astronomy and astrology are complementary studies, so that whilst astrology is less self-sufficient and factual, its employment makes the practice of astronomy more useful. Although the terms astronomia and astrologia were used interchangeably in ancient texts, this also demonstrates the early definition of two differentiated subjects which were discussed at length by Ptolemy in two separate works. Ptolemy states that having dealt with the former subject (astronomy) in its own treatise, he "shall now give an account of the second and less self-sufficient method in a properly philosophical way, so that one whose aim is the truth might never compare its perceptions with the sureness of the first". In this, and further introductory remarks, he reveals his view that astrological prediction is extremely difficult and easily subject to error, but satisfactorily attainable to those who possess the necessary skill and experience, and of too much benefit to be dismissed simply because it can sometimes be mistaken. Ptolemy's philosophical arguments Chapters 2 and 3 are important for giving Ptolemy's philosophical defense of his subject. Franz Boll noticed the arguments were paralleled in older sources, particularly those of the Stoic philosopher Posidonius (135 BCE–AD 51 BCE). Equally, Ptolemy's narrative was drawn upon by later philosophers and astronomers, such as Johannes Kepler, who used similar examples and the same order of arguments to explain the physical foundation of some astrological claims. Described as "scientifically speaking, perfectly laudable" by one modern commentator, another has condemned these chapters as the place where Ptolemy's "knowledge, intelligence and rhetorical skill" are most "misused". In chapter one Ptolemy asserts the legitimacy of the study and identifies the two main arguments