But what consciousness is, we know not; and how it is that anything so remarkable as a state of consciousness comes about as the result of irritating nervous tissue, is just as unaccountable as the appearance of the Djin when Aladdin rubbed his lamp in the story, or as any other ultimate fact of nature.

网易的翻译：

但是我们不知道意识是什么;任何如此不同寻常的意识状态都是由刺激神经组织产生的，就像阿拉丁在故事中擦灯时Djin的出现一样，或者像自然界的其他终极事实一样，都是无法解释的。

建议网友不要采纳这个翻译，这个明显就是有道自动翻译的结果，对于第二句话的主语从句中的how完全没有翻译出来，你仔细读根本不通顺。

how it is that anything so remarkable as a state of consciousness comes about as the result of irritating nervous tissue, is just as unaccountable...大致含义应该是：不同寻常的意识状态是由于刺激神经组织而产生的，但其具体的形成方式就像.......是无法解释的。

其中it是形式主语，后面的that....是真正的主语。