Columbia subject headings:
Belief and doubt--Encyclopedias.
Free thought--Encyclopedias.
Harvard subject headings:
Agnosticism--Dictionaries.
Agnosticism--Encyclopedias.
Atheism--Dictionaries.
Atheism--Encyclopedias.
Free thought--Dictionaries.
Free thought--Encyclopedias.
Rationalism--Dictionaries.
Rationalism--Encyclopedias.
Princeton subject headings:
Agnosticism--Dictionaries.
Atheism--Dictionaries.
Free thought--Dictionaries.
Rationalism--Dictionaries.