罗马尼亚数学家Gabriela Marinoschi 在Functional Approach to Nonlinear Models of Water Flow in Soils (Springer, 2006) 一书的开头，引用了他的同胞数学家Gr. C. Moisil（1906-1973）的话：纯数学家把能做的当该做的，而应用数学家把该做的当能做的——原话是... a pure mathematician does what he can do as well as he should, whilst an applied mathematician does what he should do as well as he can...——这一点区别，有点儿像为科学而科学与为“应该”而科学。可什么是“应该”呢？这又回到起点了。其实，大家都在做自己认为应该的事情，只是对“应该”有不同的认识。但因为认识不同，概念不同，所以还是“应该”需要不同的词儿来表示。

这引出一个“人生选择”：should or must？我偷换了一个概念，把can 换成了must——因为在“纯”科学看来，我们必须做我们能做的——像Hilbert大师傅的浪漫宣言一样：

Wir müssen wissen. Wir werden wissen.

（We must know. We will know.）这是希老1930年在哥尼斯堡的讲话，现在刻在他的墓碑上。

Should是别人要我做的，是被动的；而Must就不同了，它是听从自己内心的——来自旧金山的设计师Elle Luna（参与过Mailbox和Medium的设计，重新设计了iPhoneUber）说过一段很好的话，和我要说的意思一样（真的），那就听听她说的吧（见http://www.fastcompany.com/）：

Must is who we are, what we believe, and what we do when we are alone with our truest, most authentic self. It’s our instincts, our cravings and longings, the things and places and ideas we burn for, the intuition that swells up from somewhere deep inside of us. Must is what happens when we stop conforming to other people’s ideals andstart connecting to our own. Because when we choose Must, we are no longer looking for inspiration out there. Instead, we are listening to our calling from within, from some luminous, mysterious place

很多潦倒一生的人，都是为了自己的must而不在乎别人怎么看——关乎人生，自然就涉及到工作、实业和内心的呼唤（jobs, careers and callings）。Elle举例说，毕加索的艺术与生活就是融为一体的（Picasso’s life blended seamlessly with his work. It was all one huge swirling mix of bullfights and beaches and booze）。

最好的结果，当然就是那三个东西同化在一起（when our life,our essence, is one and the same with our work. It’s when job descriptions andtitles no longer make sense because we don’t go to work -- we are the work）。

当然，选择must是很艰难也很可怕的，如临悬崖而不知道悬崖下面是什么。但选择must也等于为自己找了一个能摇荡宇宙的工作：

CHOOSING MUST CREATES THE KIND OF WORK THAT PUTS RIPPLES THROUGH THE UNIVERSE.