 His whole train of thought there closely followed one that Herder had already pursued in the Critical Forests (1769), so it may be useful to begin with a brief sketch of the latter. By the time of writing the Critical Forests Herder was already committed to his own versions of principles  (4)  and  (5).  Accordingly, in reaction to the phenomenon of the non-linguistic arts the book initially set out to argue for a theory of their nature that would preserve consistency with those principles, and it did so in a very straightforward way, denying the non-linguistic arts any the ability to express thoughts or meanings autonomously of language by denying them any ability to express thoughts or meanings at all: whereas poetry has a sense, a soul, a force, music is a mere succession of objects in time, and sculpture and painting are merely spatial; whereas poetry not only depends on the senses but also relates to the imagination, music, sculpture, and painting belong solely to the senses (to hearing, feeling, and vision, respectively); whereas poetry uses voluntary and conventional signs, music, sculpture, and painting employ only natural ones. However, as Herder proceeded with his book he came to realize that this simplistic solution was untenable: in the third part of the book he stumbled upon the awkward case of ancient coins, which, though normally non-linguistic, clearly do nonetheless often express meanings and thoughts in pictorial ways. This realization did not lead him to abandon his versions of principles  (4)  and  (5),  however. Instead, it brought him to a more refined account of the non-linguistic arts which was still consistent with those principles: the non-linguistic arts do sometimes express meanings and thoughts, but the meanings and thoughts in question are ones that are parasitic on a prior linguistic expression or expressibility of them by the artist. In the fourth part of the book (which was not published until the middle of the nineteenth century, and was hence unknown to Schleiermacher) Herder already extended this solution from coins to painting; and in subsequent works he extended it to sculpture and music as well.