At the heart of Baruch Spinoza's philosophy is a challenge to the traditional Judeo-Christian view of the relationship between God and the world. While the Hebrew Bible and the Christian scriptures share a conception of God as the creator of the natural world and the director of human history, Spinoza argues that everything that exists is an aspect of God that expresses something of the divine nature. This idea that God is not separate from the world is expounded systematically in the Ethics, Spinoza's magnum opus. However, a more accessible introduction to Spinoza's view of the relationship between God and nature can be found in his discussion of miracles in an earlier text, the Theologico-Political Treatise. This book presents an innovative interpretation of the bible that undermines its authority as a source of truth, and questions the traditional understanding of prophecy, miracles and the divine law.

In chapter six of the Theologico-Political Treatise, Spinoza addresses the "confused ideas of the vulgar" on the subject of miracles. Ordinary people tend to regard apparently miraculous events – phenomena that seem to interrupt and conflict with the usual order of nature – as evidence of God's presence and activity. In fact, it is not just "the vulgar" who hold this view: throughout history, theologians have appealed to miracles to justify religious belief, and some continue to do so today.

For Spinoza, however, talk of miracles is evidence not of divine power, but of human ignorance. An event that appears to contravene the laws of nature is, he argues, simply a natural event whose cause is not yet understood. Underlying this view is the idea that God is not a transcendent being who can suspend nature's laws and intervene in its normal operations. On the contrary, "divine providence is identical with the course of nature". Spinoza argues that nature has a fixed and eternal order that cannot be contravened. What is usually, with a misguided anthropomorphism, called the will of God is in fact nothing other than this unchanging natural order.

From this it follows that God's presence and character is revealed not through apparently miraculous, supernatural events, but through nature itself. As Spinoza puts it: "God's nature and existence, and consequently His providence, cannot be known from miracles, but can all be much better perceived from the fixed and immutable order of nature."

Of course, this view has serious consequences for the interpretation of scripture, since both the Old and New Testaments include many descriptions of miraculous events. Spinoza does not simply dismiss these biblical narratives, but he argues that educated modern readers must distinguish between the opinions and customs of those who witnessed and recorded miracles, and what actually happened. Challenging the literal interpretation of scripture that prevailed in his times, Spinoza insists that "many things are narrated in Scripture as real, and were believed to be real, which were in fact only symbolic and imaginary".

This may seem reasonable enough to many contemporary religious believers, but Spinoza's attitude to the Bible was far ahead of its time. Today we take for granted a certain degree of cultural relativism, and most of us are ready to accept that ancient peoples understood the world differently from us, and therefore had different ideas about natural and divine causation. When it was first published in 1670, however, the Theologico-Political Treatise provoked widespread protest and condemnation. In fact, it was this reaction that made Spinoza decide to delay publication of the Ethics until after his death, to avoid more trouble.

But what are we to make of Spinoza's claim that God's will and natural law are one and the same thing? There are different ways to interpret this idea, some more conducive to religious belief than others. On the one hand, if God and nature are identical then perhaps the concept of God becomes dispensable. Why not simply abandon the idea of God altogether, and focus on improving our understanding of nature through scientific enquiry? On the other hand, Spinoza seems to be suggesting that God's role in our everyday lives is more constant, immediate and direct than for those who rely on miraculous, out-of-the-ordinary events as signs of divine activity.

And of course, the idea that the order of nature reveals the existence and essence of God leads straight to the view that nature is divine, and should be valued and even revered as such. In this way, Spinoza was an important influence on the 19th-century Romantic poets. Indeed, Spinoza's philosophy seems to bring together the Romantic and scientific worldviews, since it gives us reason both to love the natural world, and to improve our understanding of its laws.

• See here for Spinoza, part 1: Philosophy as a way of life