Acknowledgment
Thanks to Ericka Engelstad for insightful discussions and helpful comments on previous drafts of this paper.
Open Access
This article is distributed under the terms of the Creative Commons Attribution Noncommercial License which permits any noncommercial use, distribution, and reproduction in any medium, provided the original author(s) and source are credited
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Note
1. The first edition was published in 1940, but this study is based on the second edition