The debate over how to define feminism has raged for decades, and for two well-known, strong and successful women the answer is clear – “it’s not a choice, it’s common sense”.

Speaking at the headline event for Book Week Scotland at Stirling Castle on Thursday, First Minister Nicola Sturgeon and award-winning novelist Maggie O’Farrell explained what feminism meant to them, and the feminist books and authors that have inspired and influenced them.

Ms Sturgeon said emphatically: “Feminism is not a choice. It’s just common sense. It seems very obvious. I find any kind of injustice, irrespective of gender or race baffling. I think we have all got to go to fight any injustice.”

Ms O’Farrell said she agreed with Ms Sturgeon’s statement and shared an anecdote of how a male journalist had asked her “why are your books so female?”

Ms Sturgeon said men are never asked similar questions about their work being male-centred.

The talk initially focused on Ms O’Farrell’s body of work and her recent memoir I Am, I Am, I Am, about the 17 near-death experiences that have punctuated and defined her life.

Ms Sturgeon praised Ms O’Farrell’s writing, saying: “I am a I’m a great fan of your work... You have such strong female characters in your writing.”

Ms O’Farrell, a Northern Irish author of seven novels and the winner of the 2010 Costa Novel Award for The Hand That First Held Mine, said the title of her memoir is taken from acclaimed American writer and poet Sylvia Plath’s only novel The Bell Jar, which is about a women’s descent into madness. It comes from the sentence “I took a deep breath and listened to the old brag of my heart. I am, I am, I am”.

Two of Ms O’Farrell’s recommended favourite books also interestingly featured women who were insane or descending into insanity.

Jane Eyre by Charlotte Bronte features Mr Rochester’s wife Bertha who is described as insane throughout the novel, while 19th-century American novel The Yellow Wallpaper by Charlotte Perkins Gilman features a woman suffering from postnatal depression who ends up going mad after her physician husband prevents her from reading or working in an attempt to cure her.

Gilman’s book is considered to be a critique of the way the overwhelmingly male medical profession attempted to control and silence women.

Ms O’Farrell read Mrs Darwin from Poet Laureate Carol Ann Duffy’s book The World’s Wife, which is about the perspectives of wives of various famous men.

In it, Mrs Darwin is on a trip to the zoo with her husband Charles, who is renowned for his theory of evolution, and says to him: “Something about that chimpanzee over there reminds me of you.” The poem suggests that it was Mrs Darwin, and not her husband, who was the brains behind his work.

Ms Sturgeon said that one of the books that had most impact on her life was Sunset Song by Lewis Grassic Gibbon because it had a “really strong female character” in the form of Chris Guthrie.

Both spoke about how they were influenced by French writer and feminist Simone de Beauvoir.

Around 350 people attended the event at Stirling Castle, which sold out within 48 hours.