Since it branched off from philosophy in the 19th century, psychology has had a troubled, dual nature. Some have envisioned another natural science, one that offers causal explanations for behavior. Others have envisioned a humanistic science, one that offers context-specific descriptions of meaningfulness in human experience. The first group reduces behavior to natural mechanisms. The second insists that humanity be described in intentional or spiritual terms. Writing in the 1920s and '30s, Lev Vygotsky claimed that this split within psychology had created a crisis because it had prevented the field from gaining wide acceptance like the natural sciences. Although progress has been made over the past 70 years, Vygotsky's description rings uncomfortably true today.