The notion of a oeideaa is a key concept in early modern philosophy. From Descartesa (TM) seminal works at the beginning of the 17th century to the work of Thomas Reid in the closing years of the 18th century, discussion in theoretical philosophy is dominated by the debate about the core concept of a oeideaa . This two-volume textbook introduces eleven key authors from this period. The first volume presents the central texts in modern translation, often new translations based on the source texts. The second volume contains commentaries on each texta " with a systematic introduction, a line-by-line commentary and a contextualisation of the contents. Thus this textbook provides students of philosophy with a comprehensive overview of the modern discussion of the concept of a oeideaa .