...and I know how to get there.
Sounds existential, doesn't it?
Actually, it's the key to shifting. I've mentioned this lesson point tangentially on several occasions, and thought I'd fill in the details for easy referral in the future.
Where am I going? What note is it? What is the interval between where I start and where I end? Where is the note located - what position am I going to, and which finger am I going to use to play the note after the shift?
How am I going to get there? Whenever I muff a shift in my lesson, T- barks out "finger, string, stroke." ????????? That's shorthand for "describe to me exactly how you plan to do that shift." And there are exactly three possible answers to each component of that question - new, old, same.
Which finger are you shifting on? Old - the finger that is already down on the first note. New - the finger that will be playing the second note. Same - the same finger plays both the first (before the shift) and second (after the shift) notes, so the shift is made on that finger.
Which string are you shifting on? Same - the easiest one, when both notes are on the same string and you are shifting up or down that string. Old - shift on the first string, then cross to the new one to play the second note. New - cross strings first, then shift on the second note string.
Which bow stroke are you shifting on? Same - again the easiest choice, and the correct answer whenever the the two notes at each side of the shift are slurred. Old - shift before changing bow direction, which means the finger is in its new place and ready to play at the beginning of the next stroke. New - shift after changing bow direction, which results in a little "mew" before the second note that can be milked for an expressive slide or masked by slowing the bow speed.
Early in my sight singing study I used to whine that I could hear the note in my head, but I couldn't seem to get it out on pitch. When my teacher told me again and again, with more than a little amusement, that the reason I couldn't sing it was that I was not actually hearing it, I argued. Fruitlessly, as it turned out, but it helped pass the time until I could finally hear the note, and then sing it. Now I know that whenever I can't sing one of my pieces, it's because I don't know the notes well enough to hear them.
My shifting journey has followed a similar path. Whenever I am having difficulty with a shift, the solution is clear. If I'm missing the destination note, I make sure that I truly know where I am going by singing the note and describing it's location by position and finger. Then I make sure I know how I am going to get there - finger, string, stroke - and practice those elements slowly.
It works every time.