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Arithmetic , mereology , and a variety of other powerful logical theories could be formulated axiomatically without appeal to any more logical apparatus than first-order quantification , and this , along with Gödel and Skolem 's adherence to first-order logic , led to a general decline in work in second ( or any higher ) order logic .
 Gödel, Skolem
Please list all entity words in the text that fit the category. Output format is "word1, word2". 
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Answer:
 Gödel, Skolem