Thoughts on the Harmony of Science and Religion
One of the fundamental principles of the Baha'i Faith is that science and religion are in harmony. I understand that to mean that they present complementary aspects of creation, each having its specific domain of focus.
Abdu'l-Baha explains that there are two kinds of science and that both are necessary: "Scientific knowledge is the highest attainment upon the human plane, for science is the discoverer of realities. It is of two kinds: material and spiritual. Material science is the investigation of natural phenomena; divine science is the discovery and realization of spiritual verities. The world of humanity must acquire both. A bird has two wings; it cannot fly with one. Material and spiritual science are the two wings of human uplift and attainment. Both are necessary" 
Abdu'l-Baha further defines religion as "...consist[ing] in the necessary relationships deriving from the realities of things."  He also defines nature by saying that "By nature is meant those inherent properties and necessary relations derived from the realities of things."  Hence in the Baha'i view both science and religion are concerned with reality and the relationships and connections among the various entities in reality.
In order to understand the implications of this concept, it is necessary to understand the levels, also called kingdoms or stations, of creation. As Abdu'l-Baha explains, creation emanates from God. "It follows that all things have emanated from God; that is, it is through God that all things have been realized, and through Him that the contingent world has come to exist. The first thing to emanate from God is that universal reality which the ancient philosophers termed the 'First Intellect' and which the people of Baha'i call the 'Primal Will.”  It is also known variously as "The Word", "The Logos", and "The Holy Spirit", among other designations. This process of emanation proceeds through several levels of spirit. From the Primal Will or Holy Spirit proceeds the spiritual reality (also called heavenly spirit), then the human spirit (also called the rational soul), then the animal, vegetable, and mineral spirits. Each level of spirit contains fewer of the qualities or attributes of God and a lesser degree of consciousness; it is less perfect than the level above, and more perfect than the level below. The process of emanation occurs outside of time: it is eternal. Time itself is a product of emanation.
The process of physical creation, which occurs in time, proceeds from the bottom up. Thus the first level of spirit to transition from potential to actual is the mineral, followed by the vegetable, then the animal. These levels of spirit we know as nature. They are the object of study of science and are subject to the physical laws of nature. The higher levels, the heavenly spirit and the Holy Spirit, are the objects of study of religion. Higher levels of spirit comprehend the lower levels, but do not comprehend the levels above them. Humanity occupies a unique place as the interface between the physical realm and the spiritual realm. In the words of Abdu'l-Baha, "Man is in the ultimate degree of materiality and the beginning of spirituality; that is, he is at the end of imperfection and the beginning of perfection. He is at the furthermost degree of darkness and the beginning of the light."  Thus our bodies belong to the world of nature, and are subject to its laws and conditions. Our human spirit, or rational soul, on the other hand, belongs to the heavenly realm and is not subject to natural law. We humans are thus composite beings in that we have both spiritual and physical natures.
We see then that the domain of material science is the physical or material world, whereas the domain of spiritual science is the transcendent realm of the spirit. Different aspects of spirit are needed to understand each. The rational soul, being at a higher ontological level than the material world, can understand those worlds. However, since it is lower than the heavenly spirit, it cannot understand that world. As 'Abdu'l-Baha explains, "...the rational soul [also called the human spirit], encompasses all things and as far as human capacity permits, discovers their realities and becomes aware of the properties and effects, the characteristics and conditions of earthly things. But the human spirit, unless it be assisted by the spirit of faith, cannot become acquainted with the divine mysteries and the heavenly realities. It is like a mirror which, although clear, bright, and polished, is still in need of light. Not until a sunbeam falls upon it can it discover the divine mysteries."  Also, He states that "Yet there is a third reality in man, the spiritual reality. Through its medium one discovers spiritual revelations, a celestial faculty which is infinite as regards the intellectual as well as physical realms. That power is conferred upon man through the breath of the Holy Spirit." 
In summary then, the realities of the physical world can be discovered by "...the rational soul which is shared in common by all men, whether they be heedless or aware, wayward or faithful."  However, discovery of spiritual realities requires the power of the "spirit of faith" or "the spiritual reality", which is conferred through the breaths of the Holy Spirit. Individual people vary in their receptivity, and for that reason, there are some who maintain that only material substances and forces exist, whereas others maintain that there is also a real existing transcendent realm beyond what the senses can discover.
What, then, can we say about the harmony of science and religion? What features do they have in common? In what ways are they different? How are they complementary? Why do we need both?
Both science and religion share the conviction that the world is orderly, and provide explanations based on that belief. As Holmes Rolston III, Professor of Philosophy at Colorado State University states, "Science and religion share the conviction that the world is intelligible, susceptible to being logically understood, but they delineate this under different paradigms. In the cleanest cases we can say that science operates with the presumption that there are causes to things, religion with the presumption that there are meanings to things." 
"Causes" in science refers to the familiar concept of "cause and effect": A is said to cause B if an occurrence of A is always followed by B; A is the cause, B is the effect. When causes and effects are known, prediction is possible, and a scientific explanation of an effect is provided by delineating its causes, particularly when the causes and effects are subsumed under a covering law. An example of a covering law is the Law of Gravitation: a covering law reflects the order in the universe. In the case of gravitation, the covering law explains why objects fall, why things don't go flying off into space, why the moon orbits the earth, and so on.
"Meaning" refers to the significance, or purpose, of things and is characteristic of religion. As Rolston observes, "Occasional apprehension of meanings does not constitute a religion, any more than occasional recognition of causes constitutes a science. But where meanings are methodically detected out of a covering model, which is thought to represent an ultimate structure in reality, one has some sort of religion or one of its metaphysical cousins in philosophy. Science holds that causality runs deep in the nature of things; religion holds that what is highest in value runs deepest in the nature of things." 
It might be said, then, that science is concerned with how things happen, whereas religion is concerned with why they happen. Science concerns mechanism; religion concerns purpose. To elaborate, Abdu'l-Baha explains that "the existence of each and every thing depends upon four causes: the efficient cause, the material cause, the formal cause, and the final cause."  An example is a hammer, used to drive nails. The material cause of the hammer is wood and steel; the efficient cause is the artisan; the formal cause is the shape or form of a hammer; and the final cause is its purpose, to drive nails and to pound on things. The efficient and material causes are detectable by science, but the formal and final causes relate to purpose and meaning, which pertain to religion as broadly defined by Abdu'l-Baha above.
In order to understand a thing, we need to understand both groups (scientific and religious) of causes. If we know the material causes, we know how to make a hammer, but if we don't know anything about its form and purpose, we won't know how to use it. If we understand the form and purpose, but not the material causes, we won't know how to make one. This example is obvious and trivial, but the essential point is that both sets of causes are necessary in order to understand the hammer, which is to say that the causes are complementary. It is not the case that the scientific causes are correct, and the religious ones are mistaken; nor that the religious ones are correct but the scientific ones are mistaken. What is the case is that both are necessary for a complete understanding of the hammer. In other words, science is necessary but not sufficient, and likewise religion is necessary but not sufficient. Both together are needed, and they are complementary and in harmony.
Hammers are trivial and a poor example, but when it comes to understanding more important and subtle topics, such as whether evolution has purpose and meaning, or whether it is nothing more than a physical process devoid of meaning, the harmony of science and religion becomes essential. Science is needed to explain the mechanism of evolution, how evolution works. Religion addresses the question of whether evolution has a meaning beyond the elemental fact of its existence. Much ink has been spread on this question, with materialistic scientists arguing that evolution is just a fact and nothing more, that it is meaningless, simply a gene perpetuating its existence, thus disputing the perspective of religion; whereas some religionists claim that it has meaning, and therefore believe that they must dispute the science. In other words, both see conflict between science and religion. But there need not be, and in reality isn't a conflict, when it is understood that science tells us how evolution happens, and religion tells us what its purpose and meaning is. Conflict only arises when science attempts to intrude on the realm of religion, which is it not qualified to do; and religion tries to intrude on science, which it is also not qualified to do. The same considerations apply to other fundamental questions, such as whether the universe (creation) has a purpose or is rather something that just happened without purpose or transcendent cause.
From these considerations, it is clear that science and religion, properly understood, are complementary and harmonious.
 Abdu'l-Baha, The Promulgation of Universal Peace, p. 138
 Abdu'l-Baha, Some Answered Questions, chapter 30
 Abdu'l-Baha, Tablet to August Forel
 Some Answered Questions, chapter 53
 Some Answered Questions, chapter 55
 Some Answered Questions, chapter 64
 Abdu'l-Baha, Foundations of World Unity, p. 51
 Some Answered Questions, chapter 58
 From "Scientific and Religious Logic" in Philosophy of Religions - Selected Readings, Oxford University Press, 1966.
 Some Answered Questions, chapter 80
Last updated 11 September 2022