In this time of extraordinary pressure, educational and social, perhaps a mothers first duty to her children is to secure for them a quiet growing time, a full six years of passive receptive life, the waking part of it spent for the most part out in the fresh air. - Charlotte Mason, Home Education, Volume 1, page 42
Mason believed children ages 0-5 should not receive a formal education. To understand this more, I think it's important to look at the time in which Mason lived, and to what she was reacting.
By 1880, British law required all children between the ages of 5 and 10 to go to primary school. This was to ensure that all children had, at minimum, a basic education. Mason disagreed with this mandate.
If Mason were here today, sitting in my living room, what would she have to say about subjects such as preschool, transitional kindergarten, and 21st century kindergarten? (She had a lot to say against Victorian-era kindergarten, so my guess is that she would have more than a little to say against public kindergarten of the present.)
Mason wrote that children ages 0 through 5 should spend between four and six hours outside every day, from April through October. Her words (from Volume 3, page 44) are in bold:
Impossible! Says an
overwrought mother who sees her way to no more for her children than a
daily hour or so on the pavements of the neighbouring London squares.
Let me repeat, that I venture to suggest, not what is practicable in
any household, but what seems to me absolutely best for the children;
and that, in the faith that mothers work wonders once they are
convinced that wonders are demanded of them. A journey of twenty
minutes by rail or omnibus, and a luncheon basket, will make a day in
the country possible to most town dwellers; and if one day, why not
many, even every suitable day?
The phrase "the neighbouring London squares" could be replaced with "her urban Santa Monica neighborhood." Six hours a day? Impossible!
The mothers to whom Mason was writing had servants. They were not grocery shopping, washing dishes, or vacuuming. They had the time to take a train to the country every suitable day. For the stay-at-home parents of today, sitting on a picnic blanket in the shade of a large tree, all day, every day, just isn't an option.
The next best thing? A safe space for independent play, be it inside or out.