Some of the earliest surviving philosophical texts are the Upanishads of the later Vedic period (1000–500 BCE). Important Indian philosophical concepts include dharma, karma, samsara, moksha, and ahimsa. Indian philosophers developed a system of epistemological reasoning (pramana) and logic and investigated topics such as Ontology (metaphysics, Brahman-Atman, Sunyata-Anatta), reliable means of knowledge (epistemology, Pramanas), value system (axiology) and other topics.</ref> Indian philosophy also covered topics such as political philosophy as seen in the Arthashastra c. 4th century BCE and the philosophy of love as seen in the Kama Sutra. The Kural literature of the post-Sangam period between c. 1st century BCE and 5th century CE, written by the Tamil poet-philosopher Valluvar, is believed by many scholars to be based on Jain philosophies.