One thing that seems certain is that the Arabic version was produced in the circle of al-Kindi, a ninth century philosopher who guided the work of several translators. Endress 1973, in a groundbreaking study, showed that the Arabic Plotinus materials can be grouped linguistically and thematically with other works produced by this circle (see further below, §3). Furthermore, there is the evidence of the preface of the Theology itself. The preface begins: