Thinking is a concept derived from the universal (cross-language) semantic prime, the verb 'think', indefinable by reference to words or concepts like 'mental' or 'cognitive', or the like, which have less clear meaning and add no accuracy to the meaning of 'think', and create circularity of definition in that they themselves ultimately are defined by reference to 'think'. To quote semanticist Goddard:
It would be obscure and circular, for example, to attempt to “define” the concept of "thinking” by reference to expressions such as mental (or cognitive) processes, because terms like mental and cognitive are not any clearer than thinking in the first place. Nor does substituting these Latinate terms in place of the humble think lead to any improvement in accuracy. Quite the contrary, in fact, because there are other kinds of mental or cognitive processes aside from thinking – such as wanting, knowing, and feeling, for example. 
'Mental' and 'cognitive' characterize activities of the 'mind', originally a verb, Sanskrit, 'manyate', he thinks.
- Goddard C. (2003) Thinking across languages and cultures: six dimensions of variation. Cognitive Linguistics 14(2/3), 109-140.
- Webster's Third New International Dictionary, Unabridged. Merriam-Webster, 2002. ( 24 Mar. 2012).