Research has suggested "spiritual" people may suffer worse mental health than conventionally religious, agnostic or atheist people. But what exactly do people mean when they describe themselves as "spiritual, but not religious"?
Spirituality is a common term these days, used by Prince Charles, and by the Archbishop of York as a way of stepping beyond religious divides.
But many now call themselves "spiritual" but not religious. About a fifth of people in the UK fit into this category, according to Prof Michael King from University College London.
In the US, a Newsweek survey in 2005 put the figure at a quarter. A survey in October by the Pew Research Center suggested a lower figure with a fifth of people religiously unaffiliated and 37% of those regarding themselves as spiritual but not religious.
King's research suggested that in the UK the "spiritual" group are more likely to have mental health problems, such as anxiety or depression.
There will be people who will dispute the research, but it's certainly clear that the "spiritual, but not religious" represents a major strand of belief across the West.
It's a broad church, so to speak. The spiritually aligned range from pagans to devotees of healing crystals, among many other sub-groups.
But for millions of others it is nothing so esoteric. Instead, it's simply a "feeling" that there must be something else.
The rise of this type of spirituality has been driven by a sense that religion is out of keeping with modern values, says Mark Vernon author of How To Be an Agnostic. "People associate religious institutions with constraining doctrines, and bad things that are done in the world. That may be outright fundamentalism, the oppression of women or some kind of conflict with liberal values."
Science has replaced God for many today, Vernon suggests.
But while science may be able to explain the world, it doesn't evoke how many people feel about their place in the universe.
Awe and wonder is how spiritual people often describe their relationship with the world. There's a sense that life is more than pounds and pence, of work, childcare and the rest of the daily grind.
There are moments that seem transcendent in their lives - a beautiful sunset, a football crowd filling a stadium with noise, or a moving piece of music.
Oliver Burkeman, author of The Antidote, says the phrase "spiritual but not religious" has become a bit of a joke. But the concept is worth defending. "Spirituality I take to refer to things that are not expressible in words. There's an aspect of human experience that is non-conceptual."
It's about more than belief, Burkeman says. Just as for Christians and other religions, it is the practice of worship that is as important as the belief, he argues.
Gaetan Louis de Canonville practices mindfulness meditation in Richmond, south London. "We're not worshipping a God or paying homage to something in the sky. It's about learning to accept things like impermanence and living in the moment. If you get a glimpse of how happy you can be by embracing the moment, all the chattering of your thoughts stops."
Mike Stygal, is a secondary school teacher who practises paganism in his private life. He believes in a divine force in nature. "I believe everything is connected, I feel very in touch with nature and the changing seasons. Awe is a very good word for how I feel. It's a sense of deep respect for nature. I can communicate with the deity."
Bridget McKenzie, a cultural learning consultant, does daily walking meditations. "It's about making time to contemplate the awesomeness of life on earth, the extraordinary luck this planet has in sustaining life."
She is not a pagan but for the summer solstice organises a Garlic Man Parade in south east London to reconnect with ancient traditions. "We all sense changes in the light as the seasons change. It's important to mark the occasion."
Colin Beckley, director of the Meditation Trust, says the only true spiritual experience is silence. "Transcendence is often triggered by nature like being on a mountainside. But by learning to meditate you can bring that mountain experience to your flat in London."
Deb Hoy a practising reiki master says that by laying hands on someone according to reiki tradition a profound change can take place. "I lie in my bed and by placing my hands on different parts of the body can rebalance the energy flow of my body."
It's a physical healing practice that promotes calmness and a sense of connection with the world, she says.
Giles Terera, an actor, is not religious but is moved by everyday experiences. "When I'm abroad I love going to a church and sitting there. As much as I disagree with some of the things the Catholic Church has done, there's something very beautiful about the architecture and all the effort that that has been gone to. It's probably the same for all sacred spaces."
But for some, spirituality is a byword for irrational beliefs and a sense that anything goes.
The comedian David Mitchell mocked the tendency, writing a column imagining a spiritual summer camp. "From reflexology to astrology, from ghosts to homeopathy, from wheat intolerance to 'having a bad feeling about this', we'll be celebrating all the wild and wonderful sets of conclusions to which people the world over are jumping to fill the gap left by the retreat of organised religion."
Alan Miller, director of the thinkers' forum NY Salon, wrote that "'spiritual but not religious' offers no positive exposition or understanding or explanation of a body of belief or set of principles of any kind".
Another group of people likely to be dismissive towards the "spiritual but not religious" mindset might come from organised religion.
"People have wanted to see how they fit into the big picture, which is really fantastic," says Brian Draper, associate member of faculty at the London Institute for Contemporary Christianity. But there's a smorgasbord-like array of beliefs and many are built on "pseudo-science", he argues.
"I don't just choose spirituality as a lifestyle choice to enhance what's there, there's an element of self sacrifice to Christianity. The danger is you use spirituality as a pick and mix from consumer culture."
Humanists are deadlocked over the issue of the "spiritual" category. Andrew Copson, chief executive of the British Humanist Association, accepts that for many people it's a shorthand for saying "there must be more to life than this". But he finds its vagueness unhelpful.
"It can be used for everything from the full Catholic mass to whale songs, crystals, angels and fairies." As a humanist he prefers to avoid spirituality.
Humanism is about the belief "that human beings find value in the here and now rather than in something above and beyond". "People have social instincts and as a humanist it's about reinforcing those instincts," he explains.
The search for meaning can be exhausting. Philosopher Julian Baggini writes in The Shrink & the Sage that there is a yearning for something more. "My short reply is that you can yearn for higher as much as you like, but what you're yearning for ain't there. But the desire won't go away."
That doesn't make it a bad thing, Vernon says. But it may lead to awkward questions. And that may explain why the research finds that spiritual people have more mental health problems.
"You're going on an interior mental journey. It's risky to go and try and see things from a bigger perspective. The promise is tremendous but the journey can be very painful."
Do you consider yourself spiritual but not religious? What do you believe in?