There are various approaches to building a picture of the ‘spiritual’ entities in which indigenous Formosans believed in ‘Aboriginal Taiwan’. This article does so by studying what sources written in the seventeenth century tell us about them. One source was written by a Chinese observer, and others by two groups of Europeans: Dutch East India Company employees and Spanish missionaries. Thus, one methodological issue is that these authors looked at Formosan belief systems through the different lenses of their own religious experience and tried to fit the Formosan belief systems into their own ‘existing knowledge grids’. A related problem is that the authors’ usage of terms may differ and indeed does differ from modern usage in the anthropology of religion. Despite these methodological issues, the article argues that these sources indicate that different Formosan tribes believed in different spiritual entities and were therefore marked by their heterogeneity.