My sister sends me two WhatsApp messages each day, without fail,

At 9am: Good morning Mike and Lucy.

At 9pm: Good night Mike and Lucy.

Less frequently and less predictably she might ask,

Wot have you been doing today? or announce that, The wether is nice here.

I reply with brief, predictable messages of my own and occasionally add,

How are you feeling today?

Her answer is always the same,

I am happy today.

And, of course, she is. My sister is well cared for in her sheltered accommodation. She gets on with her flat-mates, has her own space, and exists in a mostly harmonious environment of structured freedom. Her week is a mix of routine and scheduled surprise; shopping, walking, gardening, cooking. Her needs are few and her likes extend to The Wurzels and a wide selection of DIsney and Pixar DVDs. At 47 years old, I believe that she is, as she reports, happy.

But how can I be sure? Is her 'happy' the same as her friends'? Or mine, or our parents'? How do we know if we are 'happy' or if those around us share the feeling? What, indeed, is happiness?

To find out, take your pick from the sagging shelves of happiness literature. And there are a lot of shelves to browse: The Art of Happiness; The Power of Happy; 15 Minutes to Happiness; A Monk's Guide to Happiness; The Happiness Advantage; The Happiness Hypothesis and even Bill Bailey's Remarkable Guide to Happiness.

Their messages are crisp and clear: Renounce over consumption! Simplify your life! Get out more! Notice nature! Connect with people! Give! Learn! Meditate! and, as the prescient Susan Jeffers compelled us in 1987, Feel the Fear and Do it Anyway!

Achieving happiness either clears the ground for our personal, deserved success or helps us reframe what success can be.

Social psychologist Dan Gilbert adds a pragmatic voice to the mix. In Stumbling on Happiness he argues that happiness is subjective and elusive. We can only define it for ourselves and the definition bobs about like a balloon in a storm. There are no objective criteria for 'happiness'.

Gilbert notes that our happiness is often confined to the past (nostalgia, good times) and the future (anticipation, fantasy) but that both are flawed mental activities: we misremember the past, making its recall better or worse that it actually was and we selectively edit our imagined futures. How often is your experience of an event less than you imagined it would be? Was that holiday all you dreamed of? Maybe you edited yourself out and focussed only on the sea and the palm trees; your moods, biases, personality, aches and pains oddly missing from the beach vision...

Finally, Gilbert recommends we pause before seeking our happiness - whatever we deem it to be. Do you really want that car, that house, that life, that experience? Go ask the people who already have what you think you want. What do they say about it? Maybe your happiness lies in a different direction.

So, the sad truth about happiness is that it's diverse, elusive, unpredictable and subjective. No reason not to seek it, but maybe real happiness is found in the journey not the destination.