While on a trip to Jena in the summer of 1799, Novalis met Ludwig Tieck, who became one of his closest friends and greatest intellectual influences in the last two years of his life. They became part of an informal social circle that formed around the Schlegel brothers, which has been come to be known as the Jena Romantics or "Frühromantiker" ("early romantics"). The interests of the Jena Romantics extended to philosophy as well as literature and aesthetics, and has been considered as a philosophical movement in its own right. Under the influence of Tieck, Novalis studied the works of the seventeenth-century mystic, Jakob Böhme, with whom he felt a strong affinity. He also became deeply engaged with the Platonic aesthetics of Hemsterhuis, as well as the writings of the theologian and philosopher Friedrich Schleiermacher. Schleiermacher's work inspired Novalis to write his essay, "Christenheit oder Europe" ("Christianity or Europe"), a call to return Europe to a cultural and social unity whose interpretation continues to be a source of controversy. During this time, he also wrote his poems known as "Geistliche Lieder" ("Spiritual Songs") and began his novel "Heinrich von Ofterdingen".