powerful, impregnable nation. The later ruler — the Indian Herakles — is presented as a native of India, despite similarities with the Greek Heracles. This, according to Kosmin, is because now India is shown as unconquerable. Megasthenes emphasizes that no foreign army had been able to conquer India (since Dionysus) and Indians had not invaded a foreign country either. This representation of India as an isolated, invincible country is an attempt to vindicate Seleucus' peace treaty with the Indian emperor through which he abandoned territories he could never securely hold, stabilized the East, and obtained elephants with which he could turn his attention against his great western rival, Antigonus Monophthalmus. Megasthenes states that there were no slaves in India,but the Arthashastra'' attests to the existence of slavery in contemporary India; Strabo also counters Megasthenes's claim based on a report from Onesicritus. Historian Shireen Moosvi theorizes that slaves were outcastes, and were not considered members of the society at all. According to historian Romila Thapar, the lack of sharp distinction between slaves and others in the Indian society (unlike the Greek society) may have confused Megasthenes: Indians did not use large-scale slavery as a means of production, and slaves in India could buy back their freedom or be released by their master. Megasthenes mentions seven castes in India, while the Indian texts mention only four social classes (varnas). According to Thapar, Megasthenes' categorization appears to be based on economic divisions rather than the social divisions; this is understandable because the varnas originated as economic divisions. Thapar also speculates that he wrote his account some years after his visit to India, and at this time, he "arrived at the number seven, forgetting the facts as given to him". Alternatively, it is possible that the later authors misquoted him, trying to find similarities with the Egyptian society, which according to Herodotus was divided into seven social classes. Megasthenes claims that before Alexander, no foreign power had invaded or conquered Indians, with the exception of the mythical heroes Hercules and Dionysus. However, it is known from earlier sources – such as Herodotus and the inscriptions of Darius the Great – that the Achaemenid Empire included parts of north-western part of India (present-day Pakistan). It is possible that the Achaemenid control did not extend much beyond the Indus River, which Megasthenes considered to be the border of India. Another possibility is that Megasthenes intended to