In 1824 Sadi Carnot deduced the second law of thermodynamics from a postulate that went against the future law of conservation of energy (the first law of thermodynamics). Here is an oversimplified but consonant with the quotations below presentation of (part of) Carnot's 1824 argument:

Postulate: Heat is an indestructible substance (caloric) that cannot be converted into work by the heat engine.
Consequence: A cold body accepting ALL THE HEAT taken from the warm body is ABSOLUTELY NECESSARY.

Unpublished notes written in the period 1824-1832 reveal that, after discovering the first law of thermodynamics (much earlier than the official discovery), Carnot started to doubt the necessity for a cold body:

http://www.nd.edu/~powers/ame.20231/carnot1897.pdf
REFLECTIONS ON THE MOTIVE POWER OF HEAT, Sadi Carnot: p. 225: "Heat is simply motive power, or rather motion which has changed form. It is a movement among the particles of bodies. Wherever there is destruction of motive power there is, at the same time, production of heat in quantity exactly proportional to the quantity of motive power destroyed. Reciprocally, wherever there is destruction of heat, there is production of motive power." p. 222: "Could a motion (that of radiating heat) produce matter (caloric)? No, undoubtedly; it can only produce a motion. Heat is then the result of a motion. Then it is plain that it could be produced by the consumption of motive power, and that it could produce this power. All the other phenomena - composition and decomposition of bodies, passage to the gaseous state, specific heat, equilibrium of heat, its more or less easy transmission, its constancy in experiments with the calorimeter - could be explained by this hypothesis. But it would be DIFFICULT TO EXPLAIN WHY, IN THE DEVELOPMENT OF MOTIVE POWER BY HEAT, A COLD BODY IS NECESSARY; why, in consuming the heat of a warm body, motion cannot be produced."

It seems that, almost 200 years later, Carnot's question is still relevant:

Carnot's question (asked after discovering the first law of thermodynamics): WHY, IN THE DEVELOPMENT OF MOTIVE POWER BY HEAT, A COLD BODY IS NECESSARY?

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